Cycliæ.—Palæospondylus has about 85 vertebræ.
Arthrodires.—There are about 100 vertebræ in Coccosteus.
Dipnoans.—In Protopterus there are upwards of 100 vertebræ, the last much reduced in size. Figures of Neoceratodus show about 80.
Crossopterygians.—Polypterus has 67 vertebræ; Erpetichthys, 110; Undina, about 85.
Ganoids.—In this group the numbers are also large—95 in Amia, about 55 in the short-bodied Microdon. The Sturgeons all have more than 100 vertebræ.
Soft-rayed Fishes.—Among the Teleostei, or bony fishes, those which first appear in geological history are the Isospondyli, the allies of the salmon and herring. These have all numerous vertebræ, small in size, and none of them in any notable degree modified or specialized. They abound in the depths of the ocean, but there are comparatively few of them in the tropics. The Salmonidæ which inhabit the rivers and lakes of the northern zones have from 60 to 65 vertebræ. The Myctophidæ, Stomiatidæ, and other deep-sea forms have from 40 upwards in the few species in which the number has been counted. The group of Clupeidæ is nearer the primitive stock of Isospondyli than the salmon are. This group is essentially northern in its distribution, but a considerable number of its members are found within the tropics. The common herring (Clupea harangus) ranges farther into the arctic regions than any other. Its vertebræ are 56 in number. In the shad (Alosa sapidissima), a northern species which ascends the rivers, the same number is recorded. The sprat (Clupea sprattus) and sardine (Sardinia pilchardus), ranging farther south, have from 48 to 50, while in certain small herrings (Sardinella) which are strictly confined to tropical shores the number is but 40. Allied to the herring are the anchovies, mostly tropical. The northernmost species, the common anchovy of Europe (Engraulis enchrasicolus), has 46 vertebræ. A tropical species (Anchovia browni) has 41.
There are, however, a few soft-rayed fishes confined to the tropical seas in which the numbers of vertebræ are still large, an exception to the general rule. Among these are Albula vulpes, the bonefish, with 70 vertebræ, Elops saurus, the ten-pounder, with 72, the tarpon (Tarpon atlanticus), with about 50, and the milkfish, Chanos chanos, with 72.
In a fossil Eocene herring from the Green River shales (Diplomystus) I count 40 vertebræ; in a bass-like fish (Mioplosus) from the same locality 24—these being the usual numbers in the present tropical members of these groups.
The great family of Siluridæ, or catfishes, is represented in all the fresh waters of temperate and tropical America, as well as in the warmer parts of the Old World. One division of the family, containing numerous species, abounds on the sandy shores of the tropical seas. The others are all fresh-water fishes. So far as the vertebræ in the Siluridæ have been examined, no conclusions can be drawn. The vertebræ in the marine species range from 35 to 50; in the North American forms, from 37 to 45; and in the South American fresh-water species, where there is almost every imaginable variation in form and structure, the numbers range from 28 to 50 or more. The Cyprinidæ (carp and minnows), confined to the fresh waters of the northern hemisphere, and their analogues, the Characinidæ of the rivers of South America and Africa, have also numerous vertebræ, 36 to 50 in most cases.
In general we may say of the soft-rayed fishes that very few of them are inhabitants of tropical shores. Of these few, some which are closely related to northern forms have fewer vertebræ than their cold-water analogues. In the northern species, the fresh-water species, and the species found in the deep sea the number of vertebræ is always large, but the same is true of some of the tropical species also.