276 PALEONTOLOGY. * 



IV. The fourth order, or CYCLOID, from KVK\OS, a circle, 

 have scales smooth and simple on the margin, and often 

 ornamented with various figures on their upper surface. 

 The herring and the salmon are examples. (Fig. 187.) 



As the secondary epoch is called the " age of Reptiles" 

 the primary epoch may, with equal justice, be denominated 

 the age of Fishes. They were the only vertebrate animals 

 in the ocean that deposited the palaeozoic rocks, and were the 

 tyrants of the seas of those remote periods. At the com- 

 mencement of the secondary epoch, the dominion of the 

 waters was shared between them and the ancient reptiles. 

 During the oolite period, they became subordinate to that 

 class, and their numerous genera afforded nourishment to 

 the marine Saurians, which nourished to the close of the 

 secondary epoch. 



Order PLACOID, AGASS. Have the body covered with 

 horny plates instead of scales, which are often armed with 

 a median spine, or are bristled with small eminences like a 

 rough file, as in the sliagreen of sharks. (Fig. 184.) 



The skeleton is soft and cartilaginous, and with the excep- 

 tion of the bodies of the vertebrae, the teeth, rays, and tegu- 

 mentary plates, we seldom find it preserved in a fossil state. 

 It is on the form and structure of these fragments, therefore, 

 that the fossil genera have been established by Agassiz= 



1st Family. The BAJACID^E, of which the ray is typical, 

 have the body depressed in the form of a disc, and covered 

 with spiniferous plates. The genera of this family are thus 

 distributed : PtycTiacantTius in the Devonian, Pleur -acanthus 

 in the carboniferous, and palaeozoic, Squaloraya, and Cyclar- 

 thrus in the liasic, Asterodermus, Euryarthra in the Oxford 

 stage of the oolites. Three are found in the eocene, and 

 three in the newer tertiary strata. Of these twelve genera, 

 seven are extinct, and five are living. 



2nd Family. The PBISTID^: have the saw-fish for their 

 type. The bones of the face are developed into a long pro- 

 jecting blade, armed laterally with osseous spines. An 

 extinct species of Pristis is found in the Paris beds. 



3rd Family. The CESTRACIOKTDJE have the body elon- 

 gated, the teeth flat, and embedded, like a tesselated pave- 

 ment, in the jaw. Of this family we know only one living 

 genus, Cestracion (the Port Jackson shark), and fourteen 



