926 CLASS PISCES. 



in the Cestraciont Sharks the symphysial teeth have become small 

 though prehensile, and the lateral teeth well adapted for trituration. 

 The former arrangement is particularly characteristic of modern 

 times ; the latter, it is interesting to note, attained its maximum of 

 specialisation so long ago as the Carboniferous period. In many 

 early Carboniferous genera the series of lateral crushing-teeth began 

 in part to fuse into continuous plates (Pleuroplax) ; two of these 

 plates often amalgamated (Poecilodus) ; and in the most special- 

 ised of these Cochliodonts (e.g., Deltoptychius), all traces of the 

 boundaries of the original components of the dental plates became 

 obliterated." 



Suborder i. Ichthyotomi. — This name was proposed by Pro- 

 fessor Cope for a group of primitive Elasmobranchs, ranging from 

 the Devonian to the Permian, but perhaps also surviving to the 

 Trias, and showing the following characteristic features. The endo- 

 skeleton has granular calcifications extending equally throughout 

 the cartilage ; the notochord in most, or all, cases is not constricted 

 to form distinct vertebrae ; and the calcification of its sheath in the 

 precaudal region does not extend beyond that very incomplete stage 

 to which the term rhachitomous has been applied — the explanation 

 of which is given below under the head of the Labyrinthodont Am- 

 phibians. The neural and haemal spines of the vertebrae are long 

 and slender, and no intercalary cartilages are developed. Finally, 

 the pectoral fins have a long segmented axis of the archipterygial 

 type (fig. 850). 



It may be mentioned here that Dr Koken is indisposed to admit the 

 right of the Ichthyotomi to form a group of equal rank, with that em- 

 bracing all other Elasmobranchs ; since he regards the primitive fea- 

 tures exhibited by the vertebral column, and the nature of the caudal fin, 

 as only one degree removed from those found in certain Selachii. This 

 writer, indeed, regards the Pleuraca7ithidce and Cladodontidce as so 

 closely allied to the Notidanidcs and Cestraciontidtz {HybodontidcE) that 

 he would class the whole of these families in a single group, for which 

 he proposes the name Proselachii, and in which the Cochliodontidce 

 should perhaps also be included. So far as regards the slight import- 

 ance from a classificatory point of view of the imperfect calcification of 

 the vertebral column, Dr Koken's views are in harmony with those 

 adopted below in the classification of the Labyrinthodont Amphibia. 



Family Pleuracanthid^e. — In this family the body is slender 

 and somewhat depressed ; the mouth differs from that of all the 

 Selachii in being terminal ; while the caudal fin is diphycercal. 

 There is a long and low continuous dorsal fin ; while the pectoral 

 fin has a biserial arrangement of rays somewhat after the fashion of 

 Ceratodus. The type genus Pleuracanthus has received an almost 

 bewildering number of names, of which it will suffice to mention 

 Diplodus, Orthacanthus, Xenacanthus, and Didy modus ; some of 



