The True Sharks 555 



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- 

 boniferous time. Petalodus is FlG M7 ^ i yrhizodus radicans Agas . 



equally common, but known s >z- Family Petalodontidoe. Carbon- 



iferous of Ireland. (After McCoy.) 



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 Pristodontidtz 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. 



Dasyatidae, 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 Dasyatida. 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 



