546 SHARKS AND RAYS. 



by the shortness of the thin tail, which always bears a serrated spine, and may 

 have a rudimental fin ; the minute teeth being either singly or triply cuspidate. 

 The oldest representative of the family seems to be the extinct Cyclobatis 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 backwards 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 Ichthyotomi 



and Cladodontia. 



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

 order, the characters of which have been already described ; but in the Palaeozoic 

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

 indicating two perfectly distinct ordinal groups. 



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



Group. skeleton figured on p. 317, is the lobed structure of the pectoral tins, 



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. 319. The 

 internal skeleton of these sharks shows 

 granular calcifications in the cartilage ; 

 but the notochord 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 

 teeth of a lobe-finned shark.— After Fritsch. tne example figured on p. 312. The 



upper 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 

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

 further characterised 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 ; immediately 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. 



Fold-Finned The oldest and most primitive representatives of the entire 



Group. subclass are the armoured sharks of the Devonian and lower 



Carboniferous epochs, especially characterised by the simple structure of their fins, 



