CORNIFEROUS PERIOD. 



279 



New York than those of the preceding period. In the Schoharie 

 epoch they were still farther to the east than the Cauda-Galli grit ; 

 at the same time they continue to be sandstones. But with the 

 next epoch there was a change. The continent from eastern New 



In several genera of Selachians the dorsal fin is armed at its anterior margin 

 with a large spine. In the genus Spinax (fig. 462, reduced) there are such 

 spines, one to each dorsal fin ; fig. 463 represents one of natural size for a fish 

 (Spinax) about 2i feet long. — Such spines exist also in the Cestraeionts (fig. 464), 

 the Hybodonts, and Chimseroich. In these Squaloid groups the spine is usually 

 laterally compressed, and if denticulate it is so along the posterior margin. In 

 Tryyon and some other genera among the Rays, there is a similar spine, but it 

 is flattened in a direction transverse to the body, and has both outer edges den- 

 ticulate, when either is at all so. These spines in some ancient fishes were two 

 feet or more in length (see fig. 557). In a living Cestracion, 23 inches long, it 

 is \\ inches in length. 



2. Ganoids. — The following are the principal subdivisions of Ganoids: — 



A. Peacoganoids. — In these the body has a case or coat of mail, of large 

 enamelled plates, like that of a turtle. (1.) The Coccosteids have a fish-like 

 tail, and swim by means of it: Ex., Coccosteus. (2.) The Ptericlithyds, or "winged 

 fishes" (as the term signifies), have no caudal fin for swimming, but, instead, a 

 pair of powerful paddles (pectoral fins) : Ex., Pterichthys (fig. 516). Thus there 

 are sculling and paddling Placoganoids, as well as Rays. 



B. Scale-covered Ganoids, or Lepidoganoids. — They have the body 

 covered with scales set on either like tiles or like shingles, and on this differ- 

 ence two subdivisions are based. Lepidoganoids is from Act;?, a scale, and ganoid. 



Ganoids (excepting 476, 477).— Fig. 473, Tail of Thrissops (X ]4)\ 474, Scales of Cheivolepis 

 Traillii (Xl2); 475, id. Palaxraiscus lepidurus (XG); 475 a, under-view of same; 

 476, Scale of a Cycloid; 477, id. of a Ctenoid; 478, part of pavement-teeth of Gyrodus 

 umbilicus; 479, ^ooth of Lepidosteus ; 480, id. of a Cricodus: 481, Section of tooth of 

 Lepidosteus osseus. 



(a.) Rhombifers, or Ordinary Ganoids.— The scales are rhomboidal or rect- 

 angular, bony, usually thick, shining, and enamelled, and set on like tiles (figs. 



