2922 



SHARKS AND RAYS 



half-dozen representatives from temperate and tropical seas, is characterized by the 

 great width of the disc, which is at least twice as long as wide, and also by the 

 shortness of the thin tail, which always bears a serrated spine and may have a rudi- 

 mental fin; the minute teeth being either singly or triply cuspidate. The oldest rep- 

 resentative of the family seems to be the extinct Cydobatis from the Cretaceous rocks 

 of Palestine, in which the disc is either circular or oval in form, the tail very short, 

 only slightly projecting beyond the margin of the disc, and devoid of either spine 

 or fin, while the upper surface of the body has one or more longitudinal series of 

 large spiny tubercles running backward from the pectoral girdle, the remainder of 

 the body and disc being more or less sparsely covered with minute prickles. 



THE EXTINCT LOBE-FINNED AND FOLD-FINNED SHARKS Orders 



ICHTHTOTOMI and CLADODONTIA 



The whole of the preceding members of the subclass are included in a single 

 order, the characteristics of which have been already described; but in the Paleo- 

 zoic strata of both Europe and the United States there occur remains of extinct 

 sharks, indicating two perfectly distinct ordinal groups. 



The essential characteristic of this group, as shown in the restored 

 skeleton figured on p. 2685, is the lobed structure of the pectoral fins, 

 which consist internally of a long tapering segmented axis, from 



which are given off a double series of 

 cartilaginous rays, as shown in the figure 

 on p. 2687. The internal skeleton of 

 these sharks shows granular calcifi- 

 cations in the cartilage; but the noto- 

 chord is never or but seldom constricted 

 into distinct vertebrae, the calcification, 

 except in the tail, stopping short at an 

 incomplete stage, when the body of 

 each segment of the backbone consists 

 of three separate pieces, as in the 



TEETH OF A LOBE-FINNED SHARK. 



example figured on p. 2680. The upper 



(After Fntsch.) 



and lower arches and spines of the 



backbone are tall and slender; the upper spines having no intercalary cartilages 

 between them. As represented by the genus Pleur acanthus , common to the Per- 

 mian and Carboniferous rocks of both sides of the Atlantic, these sharks are further 

 characterized by the slender and slightly-depressed form of the body, the terminal 

 position of the mouth, and the diphycercal tail. The long and low dorsal fin is 

 continued along the whole of the back from a short distance behind the head, and 

 its cartilages are more numerous than the subjacent spines of the vertebrae; imme- 

 diately behind the head is a long barbed spine, and the body was probably devoid 

 of shagreen. The teeth, as shown in the annexed illustration, are very peculiar, 

 consisting of two divergent and generally unequal-sized cones, supported on an 

 expanded base. 



