114 CHIMA2ROIDS 
IV. Gitt arcuHeEs. 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 Chimzroid and ancient 
shark may well have been evolved independently. It is 
interesting to note that in Chimzroids the spiracle is 
absent. 
V. Brain. The brain structure is archaic. Its gen- 
eral plan is, however, more shark-like than Dipnoan 
(Wilder, Ref. p. 244). 
VI. LaTERAL Ling. 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. Craspinc spine. The forehead clasper of the 
male has been a well-marked character of Chimeeroids 
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 Chimzeroids seem unquestionably nearer the stem of 
the sharks than that of other fishes. On existing evi- 
dence the Chimzroid 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 
