1 1 4 C HIM MR OIDS 



IV. Gill arches. The gills have become drawn 

 closely together as in the more highly evolved types of 

 fishes {e.g. bony fishes), and are enclosed by a protective 

 dermal flap which fringes the sides of the head. The con- 

 centration of the arches and the appearance of the dermal 

 shield suggest, however, the conditions we have seen in 

 ancient sharks (Cladoselache, Chlamydoselache, Acantho- 

 des), and cannot be given significance as the ancestral 

 form of the opercular apparatus of Teleostome. Even 

 the similar conditions of the Chimaeroid and ancient 

 shark may well have been evolved independently. It is 

 interesting to note that in Chimseroids the spiracle is 

 absent. 



V. Brain. The brain structure is archaic. Its gen- 

 eral plan is, however, more shark-like than Dipnoan 

 (Wilder, Rcf. p. 244). 



VI. Lateral line. The sensory canals possess many 

 distinctive features ; they retain their groove-like charac- 

 ter, but become widely sacculated and dilated, especially 

 in the snout region. 



VII. Clasping spine. The forehead clasper of the 

 male has been a well-marked character of Chimaeroids 

 from Liassic time. It folds anteriorly into a receptive 

 groove ; its distal end, studded with recurved spines, 

 serves in the recent forms for strongest retention. It 

 seems to represent morphologically the anterior spine of 

 a dorsal fin (cf. Pleuracanthus, p. 83). 



In spite of these differences, however, the kinships of 

 the Chimaeroids seem unquestionably nearer the stem of 

 the sharks than that of other fishes. On existing evi- 

 dence the Chimaeroid could not have been derived from 

 either Teleostome or lung-fish ; nor, on the other hand, 

 could any of the larger groups of fishes be reasonably 



