ON THE THEORY OF THE VERTEBRATE SKULL 573 



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 

 traverses 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 

 prominence, 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 P haryngobranchii), and has always, speaking 

 broadly, the same relative position with respect to the auditory 

 <;apsule and the posterior crus of the suborbitar arch. The sub- 

 orbitar arch itself is always present (except in Pliaryngobranchii) ; 

 its posterior 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- 

 minal are the same in each ; the posterior crus of the suborbitar 

 arch answers to the suspensorium of Teleostei, its anterior crus to 

 their palatopterygoid apparatus. But with all this, there are dis- 

 crepancies in the structure of the skull itself, which would forbid too 

 close an approximation between the bony and the unossified crania, if 

 their adult forms alone were examined. The study of the develop, 

 ment of the ossified vertebrate skull, however, eliminates this difficulty, 

 and satisfactorily proves that the adult crania of the lower Vertebrdta 

 are but special developments of conditions through which the em- 

 bryonic crania of the highest members of the subkingdom pass. 



^ See Note IV., on the suspensorium in fishef. 



