See:
MongoDB.Bson.Tests.Serialization.Conventions.ImmutableTypeClassMapConventionTests

Using test class:
private class TestClassD : TestClassC
{
public TestClassD(string a, string b) : base(a, b) { }
}

Will result in:
System.ArgumentOutOfRangeException
The memberInfo argument must be for class TestClassD, but was for class TestClassC.
Parameter name: memberInfo

when making the following call:
var classMap = BsonClassMap.LookupClassMap(typeof(TestClassD));