436 FISHES CHAP. 



Craniates, and from such types as these, amongst others, we may 

 reasonably look for the ancestors of all or most of the remaining 

 groups of Fishes. It has been well said of Pleuracantlius that 

 " it is a form of Fish which might with little modification become 

 either a Selachian, Dipnoan, or Crossopterygian," 1 while the con- 

 dition of the primary 



*P ^^^ upper jaw in the Chon- 



drostean Polyodon sug- 

 gests that even the more 

 primitive Actinopterygii 

 had an Elasmobranch 

 origin. The important 

 researches of Dr. Tra- 

 quair render it also 

 highly probable that the 

 ancient Ostracodermi 



FIG. 248. An embryo Shark, with its yolk-sac may claim kinship 

 (y.s) ; sp. spiracle. i /-^ i i i 



through their Coelolepid 



ancestors with some primitive type of Elasmobranch ; and within 

 the limits of the group there is ample evidence that differentia- 

 tion has taken place on many divergent lines, of which we have 

 notable examples in such specialised offshoots as the Acanthodei 

 and the Holocephali, to say nothing of several highly specialised 

 families which became extinct at successive periods in the history 

 of the group. 



Order I. Pleuropterygii. 



The only certain representative of this group is the Palaeozoic 

 form Cladoselache, probably the most primitive Elasmobranch at 

 present known (Fig. 249). Elongated and somewhat cylindrical 

 in shape, Cladoselache 2 has a terminal mouth, five or possibly 

 seven pairs of branchial clefts, and a pair of olfactory organs, 

 lateral in position near the extremity of the snout. Wide-based, 

 triangular pectoral and pelvic fins, a low anterior and a posterior 

 dorsal fin, devoid of spines, and a heterocercal caudal fin with 

 homocercal tendencies, are present, but no anal fin has yet been 



1 Smith Woodward, Vertebrate Palaeontology, Cambridge, 1898, p. 32. 



2 B. Dean, Journ. Morph. ix. 1894, p. 87. Trans. New York Acad. Sci. xiii. 1894, 

 p. 115. 



