The True Sharks 



5SS 



Fig. 347. — PnJi/rhizodus radicans Agas- 

 siz. Family Pelalodontidoe. Carbon- 

 iferous of Ireland. (After McCoy.) 



and the fins, as in the rays, are without spines. The teetli 

 bear some resemblance to those of Myliobatis. Janassa is found 

 in the coahmeasures of Europe 

 and America, and other genera 

 extend upward from the Sub- 

 carboniferous hmestones, dis- 

 appearing near the end of Car- 

 boniferous time. Petalodiis is 

 equaUy common, but known 

 only from the teeth. Other 

 widely distributed genera are Cienoptychins and Polyrhizodns. 



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 Pristodontidcc 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 Pristodns, the only genus of the group. 



Dasyatidae, or Sting-rays. — In the section Masticiira the tail 

 is slender, mostly whipdike, 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 spme 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 Dasyatidcr. 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 Urolophiis has a 



