204 Adaptations of Fishes 



Elasmobranchs . Among sharks and skates the usual num- 

 ber of segments is from 100 to 150 and upwards. In the extinct 

 species as far as known the numbers are not materially different. 

 The Carboniferous genus, Pleur acanthus, has about 115 vertebrae. 

 The Chim&ras have similar numbers; Chim&ra monstrosa has 

 about 100 in the body and more than as many more in the fila- 

 mentous tail. 



Cydicz. Palaospondylus has about 85 vertebrae. 



Arthrodires. There are about 100 vertebrae in Coccosteus. 



Dipnoans. In Protopterus there are upwards of 100 vertebrae, 

 the last much reduced in size. Figures of Neoceratodus show 

 about 80. 



Crossopterygians . Poly pier us has 67 vertebrae; Erpetichthys, 

 no; 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 vertebrae. 



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 

 vertebrae, 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 Salmonida? which inhabit the rivers and lakes of the north- 

 ern zones have from 60 to 65 vertebrae. The Myctophida, 

 Stomiatida, and other deep-sea forms have from 40 upwards 

 in the few species in which the number has been counted. 

 The group of Clupeida 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 vertebrae 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 northern- 



