419 



auditory organ, in fact, furnishes an articular surface for a strong 

 moveable peduncle, to which the hyoid arc is usually attached. At its 

 lower end, however, this peduncle does not articulate with the man- 

 dibular cartilage, but is directly connected with a strong cartilaginous 

 plate which forms the upper boundary of the gape, and is articulated 

 anteriorly with the sides of the skull in front of the orbit. This plate 

 bears the upper series of teeth, and bites more or less directly against 

 the mandible, which is moveably articulated with a condyle furnished 

 by its posterior extremity. 



The upper plate is commonly, though, as I think, erroneously, 

 regarded as the homologue of the maxilla and premaxilla in other 

 fishes ; the peduncle as the homologue of their whole suspensorium*. 



The par vagum leaves the skull behind the auditory organ ; the 

 trigeminal passes out in front of it ; and then its third division tra- 

 verses the space enclosed between the peduncle, the upper plate, 

 and the skull. The optic nerve passes through the lateral walls of 

 the skull in front of the trigeminal, and the olfactory perforates its 

 anterior boundary. 



So brief and simple a statement of the characters of the skulls of 

 these three orders of fishes, while it brings their diversities into pro- 

 minence, also exhibits an amount of uniformity among them which 

 is not a little remarkable. The exits of the great nerves have fixed 

 relations to the auditory capsules, to the anterior boundary of the 

 skull, and to the pituitary body. The inferior arc of the hyoid is 

 constant (except in the Pharyngobranchii), and has always, speaking 

 broadly, the same relative position with respect to the auditory cap- 

 sule and the posterior crus of the suborbitar arch. The suborbitar 

 arch itself is always present (except in Pharyngobranchii) ; its pos- 

 terior crus is always attached to the cranium behind the third division 

 of the trigeminal nerve, while the anterior is invariably fixed to that 

 part of the skull which lies behind, or beside, the base of the olfactory 

 capsule. 



Thus the employment of the method of gradation alone exhibits 

 a surprising uniformity in the organization of these lower forms of 

 skull ; and on comparing them with the higher forms, it seems obvious 

 that, so far as it goes, their plan is identical with that of the latter ; 

 for the relations of the auditory organ to the par vagum and trige- 

 * See Note IV., on the suspensorium in fishes. 



