Now with regard to the hard names to which we have just made allusion—names that, apart from their etymology, which is nothing more than “sending vagrant words back to their parish,” are enough to startle any one; names such as heterocercal, homocercal, cephalaspis, pterichthys, coccosteus, osteolepis, &c. &c.—why, they will all presently become plain, and, we hope, familiar to our readers. “They are,” says Hugh Miller, “like all names in science, unfamiliar in their aspect to mere English readers, just because they are names not for England alone, but for England and the world. I am assured, however, that they are all composed of very good Greek, and picturesquely descriptive of some peculiarity in the fossils they designate.”[41] The rest of this chapter will be occupied with an account of the four most remarkable and characteristic fishes of this formation, to understand which a few preliminary remarks are necessary. Cuvier divided all fish into two groups, the _bony_ and the _cartilaginous_; and these two groups he subdivided into two divisions, characterised by differences in their _fins_, or organs of locomotion, one of which he called Acanthopterygian,[42] (thorny-finned,) and the other, Malacopterygian,[43] (or soft-finned.) This concise arrangement did not, however, meet all the wants of the fish-students, and it was often practically difficult to know under which class to arrange particular specimens. More recently M. Agassiz has arranged fish, not according to their _fins_, but according to their _scales_; and simple as this classification may seem, it is one of the greatest triumphs of genius in modern times, inasmuch as all fishes extinct and existing, that have inhabited or are inhabiting the “waters under the earth,” may be grouped easily under the following four divisions:— [Illustration: 1. GANOID SCALE; as bony pike.[44] ] [Illustration: 2. PLACOID SCALE.[45] ] [Illustration: 3. CTENOID SCALE; as sole or perch.[46] ] [Illustration: 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. [Illustration: 1. HETEROCERCAL. ] [Illustration: 2. HOMOCERCAL. ] [Illustration: 3. HOMOCERCAL. ]