3. Ctenoid Scale; as sole or perch.[[46]]
4. Cycloid Scale; as herring.[[47]]
One more preliminary remark, and we will proceed to look at the four fishes already alluded to. Neither the teacher nor the student of any science can skip definitions, axioms, postulates, and so on; they must just be mastered, and their mastery is a real pleasure. In addition to a marked difference in the fins, a difference was observed also in the tails of fossil (extinct) and living pieces of fish. This difference between the tails of fish has been happily described in two words, heterocercal and homocercal, of which the figures below will give a better idea than a lengthened description.
1. Heterocercal.
2. Homocercal.
