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 Plewracanthus that 
“it is a form of Fish which might with little modification become 
either a Selachian, Dipnoan, or Crossopterygian,” * while the con- 
dition of the primary 
upper jaw in the Chon- 
drostean Polyodon sug- 
gests that even the more 
primitive Actinopterygil 
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 elaim kinship 
(y.8) 5 sp, spiracle. 
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* 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 
‘ 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, 
