PLAGIOSTOMI 113 



bodies of vertebras, showing by their extreme shortness in com- 

 parison with their breadth, by their bi-concavity, and the 

 fissures on the external surface (as shown on the lower 

 figure of cut 37), that they belonged to a shark closely allied 

 to the Porbeagle (Lamna, Cuv.) 



Family III. — Raiid^e. 

 {Bays.) 



Fossil evidences of this peculiar family of cartilaginous 

 fishes have been discovered in oolitic (Simthobatis Bclem- 

 nobatis), cretaceous, and tertiary formations, and consist of 

 defensive spines, dermal tubercles, and teeth, but chiefly 

 the latter. The most peculiar and distinctive modifications 

 of the dental system, presented by the eagle-rays (Myliobatidce) 

 are unequivocally shown by fossils of the tertiary formations, 

 and have not been found in earlier strata. 



The teeth of the rays are in general more numerous than 

 those of the sharks ; they have less mobility, are more closely 

 impacted, and in some cases are laterally united together by 

 fine sutures, so as to form a kind of mosaic pavement on both 

 the upper and lower jaws. The Myliobates, or eagle-rays, 

 which present the last-mentioned condition, unique in the ver- 

 tebrate sub-kingdom, have large and massive teeth (fig. 38) ; 

 but in the rest of the present family of cartilaginous fishes, the 

 teeth (fig. 38) are remarkable for their small size as compared 

 with those of the sharks. The teeth in some species of rays 

 are adapted for crushing, but in others they have the middle 

 or one of the angles of the crown produced into a sharp point. 

 In all genera of the ray tribe, whatever the diversity of size 

 and shape of the teeth, they are placed in several rows, and 

 succeed each other uninterruptedly from behind. 



The modification of the plagiostomous type of teeth, for the 

 purpose of crushing alimentary substances, is most complete 

 in the genus Myliobates. A view of this armature of the mouth, 



i 



