ORDER ELASMOBRANCHEI. 937 



with fig. 858 will show that the general plan of this arrangement 



of the teeth is essentially the same as in Myliobatis. This genus 



has a wide distribution, being 



found in the Upper Cretaceous 



of Europe, India, and North 



America. 



Family Trygonid^. — In this 

 family the pectoral fins continue 

 to the extremity of the muzzle, with 

 which they -are confluent. The 

 tail is slender, and sharply distin- 

 guished from the disk ; while the 

 vertical fins are imperfect or ab- „. _, " . . 



r big. 800. — Diagram of the arrangement 



Sent, and may be replaced by of the teeth in the upper (a) and lower (b) 



i . ry-ii • jaws of Ptychodus decurrcns. (After S. 



serrated spines. lnere is some woodward.) Reduced, 

 uncertainty as to the occurrence of 



Trygon in a fossil state, but it is probable that either this genus or 

 the allied Tmiiura date from the Middle Eocene of Monte Bolca, 

 and remains referred to the former have been described from the 

 Upper Cretaceous of New Zealand. The extinct Xifihotrygon, from 

 the Eocene of Wyoming, is distinguished by its cuspidate teeth. 

 Remains referable to the existing tropical genus Urolophus occur in 

 the Middle Eocene of Italy, and perhaps in that of Belgium ; while 

 Cyclobatis is an extinct genus from the Cretaceous of the Lebanon. 



Section B. Asterospondyli. — This section is distinguished from 

 the last by having the radiating laminae predominating over the con- 

 centric ones in the fully calcified vertebral centra, so that a section 

 shows a star-like arrangement. The anal fin is always present ; 

 while specialisation does not tend to a flattening of the body, or to 

 an expansion and forward growth of the pectoral fins, and the spir- 

 acles are small, and may disappear. This section comprises the 

 true Sharks and the Dog-fishes. 



Ail these fishes have elongated and subcylindrical bodies, and a 

 strong tail, well adapted for swimming. The anterior, and very fre- 

 quently all the teeth are formed on the type of a laterally compressed 

 cone with trenchant edges, at the base of which two or more minor 

 cones may be developed. In many cases, however, the hinder teeth 

 have obtuse crowns, adapted for crushing. The two rami of the jaws 

 are never parallel to one another, in consequence of which the teeth 

 are always set in oblique rows (fig. 864), and never form the straight 

 antero-posterior rows, like those occurring in the Rays (fig. 860). Sharks 

 are carnivorous, and of active pelagic habits ; and are most numerous 

 in tropical seas, although they ascend tidal rivers, and are even found in 

 an inland lake in the Fiji Islands. 



This section may be divided into two series, in the first of which 



