diff --git a/src/sexpression/LispNumber.java b/src/sexpression/LispNumber.java
index 68d9853..a34e6c0 100644
--- a/src/sexpression/LispNumber.java
+++ b/src/sexpression/LispNumber.java
@@ -1,5 +1,9 @@
 package sexpression;
 
+import java.text.MessageFormat;
+
+import error.LispException;
+
 public class LispNumber extends Atom {
 
     private int value;
@@ -10,7 +14,7 @@ public class LispNumber extends Atom {
         try {
             this.value = Integer.parseInt(text);
         } catch (NumberFormatException e) {
-            throw new IllegalArgumentException(text + " is not a valid integer");
+            throw new InvalidNumberException(text);
         }
     }
 
@@ -28,4 +32,25 @@ public class LispNumber extends Atom {
         return value;
     }
 
+    public class InvalidNumberException extends LispException {
+
+        private static final long serialVersionUID = 1L;
+        
+        private String text;
+        
+        public InvalidNumberException(String text) {
+            this.text = text;
+        }
+
+        @Override
+        public int getSeverity() {
+            return 0;
+        }
+        
+        @Override
+        public String getMessage() {
+            return MessageFormat.format("{0} is not a valid integer", text);
+        }
+    }
+
 }
diff --git a/src/sexpression/SExpression.java b/src/sexpression/SExpression.java
index 28a7c4e..cb86b15 100644
--- a/src/sexpression/SExpression.java
+++ b/src/sexpression/SExpression.java
@@ -2,20 +2,6 @@ package sexpression;
 
 public abstract class SExpression {
 
-    // for mark and sweep garbage collection
-    private boolean marked = false;
-
-    public final boolean isMarked() {
-        return marked;
-    }
-
-    public final void setMarked(final boolean value) {
-        marked = value;
-    }
-
-    // Lisp type predicates...
-    // overridden in subclasses to describe their Lisp type
-
     public boolean nullp() {
         return false;
     }
diff --git a/src/token/Eof.java b/src/token/Eof.java
index 6e8b22b..9302858 100644
--- a/src/token/Eof.java
+++ b/src/token/Eof.java
@@ -3,8 +3,8 @@ package token;
 import java.util.function.Supplier;
 
 import file.FilePosition;
-import sexpression.MalformedSExpressionException.EofEncounteredException;
 import sexpression.SExpression;
+import token.ParseException.EofEncounteredException;
 
 public class Eof extends Token {
 
diff --git a/src/sexpression/MalformedSExpressionException.java b/src/token/ParseException.java
similarity index 63%
rename from src/sexpression/MalformedSExpressionException.java
rename to src/token/ParseException.java
index e2966e2..7c765ce 100644
--- a/src/sexpression/MalformedSExpressionException.java
+++ b/src/token/ParseException.java
@@ -1,16 +1,15 @@
-package sexpression;
+package token;
 
 import java.text.MessageFormat;
 
 import error.LispException;
-import token.Token;
 
-public abstract class MalformedSExpressionException extends LispException {
+public abstract class ParseException extends LispException {
 
     private static final long serialVersionUID = 1L;
     private Token token;
 
-    public MalformedSExpressionException(Token token) {
+    public ParseException(Token token) {
         this.token = token;
     }
 
@@ -27,7 +26,7 @@ public abstract class MalformedSExpressionException extends LispException {
 
     public abstract String getMessagePrefix();
 
-    public static class EofEncounteredException extends MalformedSExpressionException {
+    public static class EofEncounteredException extends ParseException {
 
         private static final long serialVersionUID = 1L;
 
@@ -40,7 +39,7 @@ public abstract class MalformedSExpressionException extends LispException {
         }
     }
 
-    public static class StartsWithRightParenthesisException extends MalformedSExpressionException {
+    public static class StartsWithRightParenthesisException extends ParseException {
 
         private static final long serialVersionUID = 1L;
 
@@ -53,17 +52,4 @@ public abstract class MalformedSExpressionException extends LispException {
         }
     }
 
-    public static class UnrecognizedTokenException extends MalformedSExpressionException {
-
-        private static final long serialVersionUID = 1L;
-
-        public UnrecognizedTokenException(Token token) {
-            super(token);
-        }
-
-        public String getMessagePrefix() {
-            return "Unrecognized token";
-        }
-    }
-
 }
\ No newline at end of file
diff --git a/src/token/RightParenthesis.java b/src/token/RightParenthesis.java
index 135788c..fca5cc3 100644
--- a/src/token/RightParenthesis.java
+++ b/src/token/RightParenthesis.java
@@ -3,9 +3,9 @@ package token;
 import java.util.function.Supplier;
 
 import file.FilePosition;
-import sexpression.MalformedSExpressionException.StartsWithRightParenthesisException;
 import sexpression.Nil;
 import sexpression.SExpression;
+import token.ParseException.StartsWithRightParenthesisException;
 
 public class RightParenthesis extends Token {
 
diff --git a/test/parser/LispParserTester.java b/test/parser/LispParserTester.java
index 16f13ae..8d961dd 100644
--- a/test/parser/LispParserTester.java
+++ b/test/parser/LispParserTester.java
@@ -1,6 +1,7 @@
 package parser;
 
 import static org.junit.Assert.*;
+import static parser.SExpressionTypeAssertions.*;
 import static testutil.TestUtilities.createIOExceptionThrowingInputStream;
 import static testutil.TestUtilities.createInputStreamFromString;
 
@@ -8,13 +9,12 @@ import java.io.InputStream;
 
 import org.junit.Test;
 
+import error.ErrorManager;
 import error.LispException;
 import scanner.LispInputStream.UncheckedIOException;
 import scanner.LispScanner.UnterminatedStringException;
-import sexpression.MalformedSExpressionException.EofEncounteredException;
-import sexpression.MalformedSExpressionException.StartsWithRightParenthesisException;
-import sexpression.Nil;
-import sexpression.SExpression;
+import token.ParseException.EofEncounteredException;
+import token.ParseException.StartsWithRightParenthesisException;
 import token.TokenFactory.BadCharacterException;
 
 public class LispParserTester {
@@ -29,64 +29,6 @@ public class LispParserTester {
         return new LispParser(stringInputStream, "testFile");
     }
 
-    private void assertList(SExpression sExpression) {
-        assertFalse(sExpression.atomp());
-        assertTrue(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertTrue(sExpression.listp());
-        assertFalse(sExpression.nullp());
-        assertFalse(sExpression.numberp());
-        assertFalse(sExpression.stringp());
-        assertFalse(sExpression.symbolp());
-    }
-
-    private void assertNil(SExpression sExpression) {
-        assertEquals(sExpression, Nil.getUniqueInstance());
-
-        assertTrue(sExpression.atomp());
-        assertFalse(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertTrue(sExpression.listp());
-        assertTrue(sExpression.nullp());
-        assertFalse(sExpression.numberp());
-        assertFalse(sExpression.stringp());
-        assertTrue(sExpression.symbolp());
-
-    }
-
-    private void assertNumber(SExpression sExpression) {
-        assertTrue(sExpression.atomp());
-        assertFalse(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertFalse(sExpression.listp());
-        assertFalse(sExpression.nullp());
-        assertTrue(sExpression.numberp());
-        assertFalse(sExpression.stringp());
-        assertFalse(sExpression.symbolp());
-    }
-
-    private void assertString(SExpression sExpression) {
-        assertTrue(sExpression.atomp());
-        assertFalse(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertFalse(sExpression.listp());
-        assertFalse(sExpression.nullp());
-        assertFalse(sExpression.numberp());
-        assertTrue(sExpression.stringp());
-        assertFalse(sExpression.symbolp());
-    }
-
-    private void assertSymbol(SExpression sExpression) {
-        assertTrue(sExpression.atomp());
-        assertFalse(sExpression.consp());
-        assertFalse(sExpression.functionp());
-        assertFalse(sExpression.listp());
-        assertFalse(sExpression.nullp());
-        assertFalse(sExpression.numberp());
-        assertFalse(sExpression.stringp());
-        assertTrue(sExpression.symbolp());
-    }
-
     @Test
     public void testEofMethod_ReturnsTrueWithNoInput() {
         String input = "";
@@ -107,8 +49,8 @@ public class LispParserTester {
     public void testEofMethod_ReturnsTrueAfterSomeInput() {
         String input = "(yyz 9 9 9)";
         LispParser parser = createLispParser(input);
-        parser.getNextSExpression();
 
+        parser.getNextSExpression();
         assertTrue(parser.isEof());
     }
 
@@ -116,6 +58,7 @@ public class LispParserTester {
     public void testEofMethod_ReturnsFalseAfterMultipleExpressions() {
         String input = "()()()";
         LispParser parser = createLispParser(input);
+
         assertFalse(parser.isEof());
         parser.getNextSExpression();
         assertFalse(parser.isEof());
@@ -127,6 +70,7 @@ public class LispParserTester {
     public void testEofMethod_ReturnsTrueAfterMultipleExpressions() {
         String input = "()()()";
         LispParser parser = createLispParser(input);
+
         assertFalse(parser.isEof());
         parser.getNextSExpression();
         assertFalse(parser.isEof());
@@ -238,6 +182,33 @@ public class LispParserTester {
         parser.getNextSExpression();
     }
 
+    @Test
+    public void givenBadToken_ExceptionHasCorrectSeverity() {
+        String input = "[";
+        LispParser parser = createLispParser(input);
+
+        try {
+            parser.getNextSExpression();
+        } catch (BadCharacterException e) {
+            assertTrue(e.getSeverity() < ErrorManager.CRITICAL_LEVEL);
+        }
+    }
+
+    @Test
+    public void givenBadToken_ExceptionHasMessageText() {
+        String input = "[";
+        LispParser parser = createLispParser(input);
+
+        try {
+            parser.getNextSExpression();
+        } catch (BadCharacterException e) {
+            String message = e.getMessage();
+
+            assertNotNull(message);
+            assertTrue(message.length() > 0);
+        }
+    }
+
     @Test(expected = UnterminatedStringException.class)
     public void givenUnterminatedString_ThrowsException() {
         String input = "\"string";
@@ -254,6 +225,33 @@ public class LispParserTester {
         parser.getNextSExpression();
     }
 
+    @Test
+    public void givenUnterminatedList_ExceptionHasCorrectSeverity() {
+        String input = "(bad list";
+        LispParser parser = createLispParser(input);
+
+        try {
+            parser.getNextSExpression();
+        } catch (EofEncounteredException e) {
+            assertTrue(e.getSeverity() < ErrorManager.CRITICAL_LEVEL);
+        }
+    }
+
+    @Test
+    public void givenUnterminatedList_ExceptionHasMessage() {
+        String input = "(bad list";
+        LispParser parser = createLispParser(input);
+
+        try {
+            parser.getNextSExpression();
+        } catch (EofEncounteredException e) {
+            String message = e.getMessage();
+
+            assertNotNull(message);
+            assertTrue(message.length() > 0);
+        }
+    }
+
     @Test(expected = StartsWithRightParenthesisException.class)
     public void givenUnmatchedRightParenthesis_ThrowsException() {
         String input = ")";
@@ -262,6 +260,33 @@ public class LispParserTester {
         parser.getNextSExpression();
     }
 
+    @Test
+    public void givenUnmatchedRightParenthesis_ExceptionHasCorrectSeverity() {
+        String input = ")";
+        LispParser parser = createLispParser(input);
+
+        try {
+            parser.getNextSExpression();
+        } catch (StartsWithRightParenthesisException e) {
+            assertTrue(e.getSeverity() < ErrorManager.CRITICAL_LEVEL);
+        }
+    }
+
+    @Test
+    public void givenUnmatchedRightParenthesis_ExceptionHasMessage() {
+        String input = ")";
+        LispParser parser = createLispParser(input);
+
+        try {
+            parser.getNextSExpression();
+        } catch (StartsWithRightParenthesisException e) {
+            String message = e.getMessage();
+
+            assertNotNull(message);
+            assertTrue(message.length() > 0);
+        }
+    }
+
     @Test(expected = BadCharacterException.class)
     public void givenBadCharacter_ThrowsExceptionAfterEofCalled() {
         String input = "[";
diff --git a/test/parser/SExpressionTypeAssertions.java b/test/parser/SExpressionTypeAssertions.java
new file mode 100644
index 0000000..a173b8f
--- /dev/null
+++ b/test/parser/SExpressionTypeAssertions.java
@@ -0,0 +1,68 @@
+package parser;
+
+import static org.junit.Assert.*;
+
+import sexpression.Nil;
+import sexpression.SExpression;
+
+public final class SExpressionTypeAssertions {
+
+    public static void assertList(SExpression sExpression) {
+        assertFalse(sExpression.atomp());
+        assertTrue(sExpression.consp());
+        assertFalse(sExpression.functionp());
+        assertTrue(sExpression.listp());
+        assertFalse(sExpression.nullp());
+        assertFalse(sExpression.numberp());
+        assertFalse(sExpression.stringp());
+        assertFalse(sExpression.symbolp());
+    }
+
+    public static void assertNil(SExpression sExpression) {
+        assertEquals(sExpression, Nil.getUniqueInstance());
+
+        assertTrue(sExpression.atomp());
+        assertFalse(sExpression.consp());
+        assertFalse(sExpression.functionp());
+        assertTrue(sExpression.listp());
+        assertTrue(sExpression.nullp());
+        assertFalse(sExpression.numberp());
+        assertFalse(sExpression.stringp());
+        assertTrue(sExpression.symbolp());
+
+    }
+
+    public static void assertNumber(SExpression sExpression) {
+        assertTrue(sExpression.atomp());
+        assertFalse(sExpression.consp());
+        assertFalse(sExpression.functionp());
+        assertFalse(sExpression.listp());
+        assertFalse(sExpression.nullp());
+        assertTrue(sExpression.numberp());
+        assertFalse(sExpression.stringp());
+        assertFalse(sExpression.symbolp());
+    }
+
+    public static void assertString(SExpression sExpression) {
+        assertTrue(sExpression.atomp());
+        assertFalse(sExpression.consp());
+        assertFalse(sExpression.functionp());
+        assertFalse(sExpression.listp());
+        assertFalse(sExpression.nullp());
+        assertFalse(sExpression.numberp());
+        assertTrue(sExpression.stringp());
+        assertFalse(sExpression.symbolp());
+    }
+
+    public static void assertSymbol(SExpression sExpression) {
+        assertTrue(sExpression.atomp());
+        assertFalse(sExpression.consp());
+        assertFalse(sExpression.functionp());
+        assertFalse(sExpression.listp());
+        assertFalse(sExpression.nullp());
+        assertFalse(sExpression.numberp());
+        assertFalse(sExpression.stringp());
+        assertTrue(sExpression.symbolp());
+    }
+
+}
diff --git a/test/sexpression/SExpressionTester.java b/test/sexpression/SExpressionTester.java
index 3216773..7f9cd9b 100644
--- a/test/sexpression/SExpressionTester.java
+++ b/test/sexpression/SExpressionTester.java
@@ -1,10 +1,13 @@
 package sexpression;
 
-import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.*;
 
 import org.junit.Before;
 import org.junit.Test;
 
+import error.ErrorManager;
+import sexpression.LispNumber.InvalidNumberException;
+
 public class SExpressionTester {
 
     private void assertSExpressionMatchesString(String expected, SExpression sExpression) {
@@ -67,4 +70,72 @@ public class SExpressionTester {
         assertSExpressionMatchesString(expected, list);
     }
 
+    @Test
+    public void testConsWithNonListCdrToString() {
+        String expected = "(A . B)";
+        Cons list = new Cons(new Symbol("A"), new Symbol("B"));
+
+        assertSExpressionMatchesString(expected, list);
+    }
+
+    @Test
+    public void testCarOfNilIsNil() {
+        assertEquals(Nil.getUniqueInstance().getCar(), Nil.getUniqueInstance());
+    }
+
+    @Test
+    public void testCdrOfNilIsNil() {
+        assertEquals(Nil.getUniqueInstance().getCdr(), Nil.getUniqueInstance());
+    }
+
+    @Test
+    public void afterSettingCarOfNil_ShouldStillBeNil() {
+        Cons nil = Nil.getUniqueInstance();
+        nil.setCar(new LispNumber(2));
+
+        assertEquals(nil.getCar(), Nil.getUniqueInstance());
+    }
+
+    @Test
+    public void afterSettingCdrOfNil_ShouldStillBeNil() {
+        Cons nil = Nil.getUniqueInstance();
+        nil.setCdr(new LispNumber(2));
+
+        assertEquals(nil.getCdr(), Nil.getUniqueInstance());
+    }
+
+    @Test
+    public void testNumberValue() {
+        int value = 12;
+        LispNumber number = new LispNumber(String.valueOf(value));
+
+        assertEquals(number.getValue(), value);
+    }
+
+    @Test(expected = InvalidNumberException.class)
+    public void testInvalidNumberText_ThrowsException() {
+        new LispNumber("a");
+    }
+
+    @Test
+    public void testInvalidNumberException_HasCorrectSeverity() {
+        try {
+            new LispNumber("a");
+        } catch (InvalidNumberException e) {
+            assertTrue(e.getSeverity() < ErrorManager.CRITICAL_LEVEL);
+        }
+    }
+
+    @Test
+    public void testInvalidNumberException_HasMessageText() {
+        try {
+            new LispNumber("a");
+        } catch (InvalidNumberException e) {
+            String message = e.getMessage();
+
+            assertNotNull(message);
+            assertTrue(message.length() > 0);
+        }
+    }
+
 }
diff --git a/test/testutil/TestUtilities.java b/test/testutil/TestUtilities.java
index 97e4b27..4081007 100644
--- a/test/testutil/TestUtilities.java
+++ b/test/testutil/TestUtilities.java
@@ -2,7 +2,7 @@ package testutil;
 
 import java.io.*;
 
-public class TestUtilities {
+public final class TestUtilities {
 
     public static InputStream createInputStreamFromString(String string) {
         return new ByteArrayInputStream(string.getBytes());