

264 



PALEOZOIC TIME. 



is the most ancient kind of fish known. From the name of the genus 

 Oephalaspis (signifying shield-like head), the group is called Cephal- 

 aspids. Two of these head-shields are figured among Upper Silurian 

 fish-remains, on page 246, and the form of an entire British Devonian 

 specimen, in Fig. 568, on page 286. The only species of the kind in 

 the American Corniferous is a Cephalaspis, from Gaspe, on the Gulf 

 cf St. Lawrence. 



Figs. 513-521. 



518. 



Ganoids (excepting 516, 517). — Fig. 513, Tail of Thrissops (XK); 514, Scales of Cheirolepis 

 Traillii (Xl2); 515, id. Palaecmiscus lepidurus (X 6) ; 515 a, under-view of same; 516, Scale of 

 a Cycloid ; 517, id. of a Ctenoid ; 518, part of pavement- teeth of Gyrodus umbilicus ; 519, Tooth 

 of Lepidosteus ; 520, id. of a Cricodus ; 521, Section of tooth of Lepidosteus osseus. 



The teeth in these Ganoids are often large and conical. Two of 

 them are represented in Figs. 519, 520 ; they are furrowed vertically, 

 and have an internal labyrinthine structure, as represented (in one of its 

 simpler varieties), in Fig. 521, a view of a transverse section enlarged. 



The tails of the ancient Ganoids were vertebrated, that is, the 

 vertebral column extended nearly or quite to the extremity ; generally 

 following the course of the upper lobe of the caudal fin, as in Fig 

 570, p. 286, and Fig. 696, p. 371, but sometimes terminating at the 

 extremity of the middle of the tail (p. 336). In modern Ganoids, on 

 the contrary, as in ordinary fishes, the vertebral column stops at the 

 commencement of the caudal fin, as in Fig. 513. Five genera of 

 living Ganoids {Lepidosteus or Gar-pike, Amia, Accipenser or Sturgeon, 

 Spatularia, and Scaphirhynchus), belong to North America, and the 

 other two known are African. 



3. Placoderms, fishes having the body partly or wholly covered by 

 bony plates, turtle-like, and named from 7r\d£, plate, and Sipfxa, skin, 

 with which the Cephalaspids are often united. It is questioned 

 whether they were more nearly related to the sharks or to the 

 Ganoids. Two foreign species of Placoderms are represented, reduced, 

 in Figs. 566, 567, p. 285. Newberry has described a dorsal plate of 

 a related fish, found in the Ohio Corniferous, which has a length and 

 breadth of eight inches. 



