. Fishes. Fishes. 212 The True Sharks. Fig. 153.âPoli/rhizodus radicans Agas- siz. Family Petalodontidae. Carbon- " " â " (After McCoy.) iferous of Ireland. and the fins, as in tlie 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. Petalodits is equally common, but known only from the teeth. Other widely distributed genera are Ctenoptychius and Polyrhizodus. These f
. Fishes. Fishes. 212 The True Sharks. Fig. 153.âPoli/rhizodus radicans Agas- siz. Family Petalodontidae. Carbon- " " â " (After McCoy.) iferous of Ireland. and the fins, as in tlie 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. Petalodits is equally common, but known 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 Pristodontidce 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. Dasyatidse, or Sting-rays.âIn the section Masticura 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 DasyatidcB. This group is characterized by the small skate-li
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