212 The True Sharks 
and the fins, as in the rays, are without spines. The teeth 
bear some resemblance to those of Myliobatis. Janassa is found 
in the coal-measures of Europe 
and America, and other genera 
extend upward from the Sub- 
carboniferous limestones, dis- 
appearing near the end of Car- UW WSoyy 
boniferous time. Petalodus is Fig. 153.--Polyrhizodus radicans Agas- 
equally common, but known $i Family, Pilodonam ay 
only from the teeth. Other 
widely distributed genera are Ctenoptychius and Polyrhizodus. 
These forms may be intermediate between the skates and 
the sting-rays. In dentition they resemble most the latter. 
Similar to these is the extinct family of Pristodontide with 
one large tooth in each jaw, the one hollowed out to meet the 
other. It is supposed that but two teeth existed in life, but 
that is not certain. Nothing is known of the rest of the body 
in Pristodus, the only genus of the group. 
Dasyatide, or Sting-rays.—In the section Masticura the tail 
is slender, mostly whip-like, without rayed dorsal or caudal 
fins, and it is usually armed with a very long spine with saw- 
teeth projecting backward. In the typical forms this is a 
very effective weapon, being wielded with great force and making 
a jagged wound which in man rarely heals without danger of 
blood-poisoning. There is no specific poison, but the slime 
and the loose cuticle of the spine serve to aggravate the irregu- 
lar cut. I have seen one sting-ray thrust this spine through 
the body of another lying near it in a boat. Occasionally two 
or three of these spines are present. In the more specialized 
forms of sting-rays this spine loses its importance. It be- 
comes very small and not functional, and is then occasionally 
or even generally absent in individuals. 
The common sting-rays, those in which the caudal spine 
is most developed, belong to the family of Dasyatide. This 
group is characterized by the small skate-like teeth and by 
the non-extension of the pectoral rays on the head. The skin is 
smooth or more or less rough. These animals lie flat on the sandy 
bottoms in nearly all seas, feeding on crabs and shellfish. All 
hatch the eggs within the body. The genus Urolophus has a 
