PISCES. 



73 



No. 122. Ptychodus decurrens, Agassiz. 

 Two Teeth. The teeth of this cestraciont are large and quadrangular. 



The crown is deeper than the root, and the margin is granulated, 

 chalk of Kent, England. 



From the 



No. 123. [330] Squatina acanthoderma, Fraas. 



Body and Head, on slab (cast). This ex- 

 tinct Placold approached the Ray family, by 

 the situation of the eyes on the dorsal face 

 and by the development of the pectoral fins. 

 The mouth was at the end of the muzzle. 

 The contour and outer border of the body 

 has left a full impression on the stone. This 

 fine fossil was discovered in the Lithographic 

 Slate (Upper Oolite), at Eichstadt, Bavaria, 

 and is in the Tylerian Museum at Haarlem, 

 Holland. 



Size, 2 ft. 4 in. X 1 ft. 8 in. 



ORDER TELEOSTEI. 



The great majority of living iislies, and nearly all of the more 

 familiar species belong in this order. No forms are positively 

 known earlier than the Cretaceous, since which time the group 

 has been continually increasing. 



The Teleosts may be broadly described by the following char- 

 acters : The vertebra-skeleton is well ossified, and the skull con- 

 sists of distinct bones ; a lower jaw is always present ; the limbs, 

 which are usually present, are represented by fins ; the gills are 

 free and comb-like and protected by a bony cover (operculum) ; 

 the heart has an auricle and ventricle, and the hulhus arteriosus 

 is not regularly contractile. The scales of the teleosts may be 

 ganoid, but usually are imbricated, iinenameled horny plates, 

 developed in a fold of the skin ; those with plain margins being 

 called " cycloid," those with the hinder margin fringed with 

 spines, or comb-like, being called " ctenoid." The nasal sacs 

 never communicate with the throat. 



