206 The True Sharks 
crowded downward to the under side of the body-disk. As 
fossil, Pristiophorus is known only from a few detached verte- 
bre found in Germany. 
Suborder Batoidei, or Rays.—The suborder of Batoidei, Raje, 
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 Squatinide (angel-fishes) and 
the Pristiophoride (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 vertebree. 
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. 
Pristidide, or Sawfishes.—The sawfishes, Pristidide, are long, 
shark-like rays of large size, having, like the sawsharks, the 
