The True Sharks 549 



crowded downward to the under side of the body-disk. As 

 fossil, Pristiophorus is known only from a few detached verte- 

 brae found in Germany. 



Suborder Batoidei, or Rays. The suborder of Batoidei, Raj<z, 

 or Hypotrema, including the skates and rays, is a direct modern 

 offshoot from the ancestors of tectospondylous sharks, its char- 

 acters all specialized in the direction of life on the bottom with 

 a food of shells, crabs, and other creatures less active than fishes. 



The single tangible distinctive character of the rays as a 

 whole lies in the position of the gill-openings, which are directly 

 below the disk and not on the side of the neck in all the sharks. 

 This difference in position is produced by the anterior encroach- 

 ment of the large pectoral fins, which are more or less attached to 

 the side of the head. By this arrangement, which aids in giving 

 the body the form of a flat disk, the gill-openings are limited 

 and forced downward. In the Squatinida (angel-fishes) and 

 the Pristiophoridce (sawsharks) the gill-openings have an inter- 

 mediate position, and these families might well be referred to 

 the Batoidei, with which group they agree in the tectospondy- 

 lous vertebrae. 



Other characters of the rays, appearing progressively, are 

 the widening of the disk, through the greater and greater de- 

 velopment of the fins, the reduction of the tail, which in the 

 more specialized forms becomes a long whip, the reduction, more 

 and more posterior insertion, and the final loss of the dorsal 

 fins, which are always without spine, the reduction of the teeth 

 to a tessellated pavement, then finally to flat plates and the 

 retention of the large spiracle. Through this spiracle the rays 

 breathe while lying on the bottom, thus avoiding the danger of 

 introducing sand into their gills, as would be done if they 

 breathed through the mouth. In common with the cyclospon- 

 dylous sharks, all the rays lack the anal fin. The rays rarely 

 descend to great depths in the sea. The different members 

 have varying relations, but the group most naturally divides 

 into thick-tailed rays or skates (Sarcura) and whip-tailed rays 

 or sting-rays (Masticura). The former are much nearer to the 

 sharks and also appear earliest in geological times. 



Pristididae, or Sawfishes. The sawfishes, Pristidida, are long, 

 shark-like rays of large size, having, like the sawsharks, the 



