170 ON THE MOEPHOLOGICAL CONSTITUTION OF 



become involved in, or have taken the place of, its neural 

 chamber, and have therefore their inner orifices or posterior 

 nares directed downwards, on the central aspect. 



The mode in which the walls and cavities of the olfactory 

 capsules of the bird become involved or lost in its ethmoidal 

 neural chamber and walls, may be morphologically conceived, 

 if the structure is compared with the corresponding segment 

 of the cranium of a ray. The cranium of the plagiostome is 

 modelled on the form of the "primordial cranium" of the 

 mammal and bird. The laterally projecting sessile cartila- 

 ginous olfactory cups communicate each by a wide orifice with 

 the cranial cavity. If the orifices be conceived as much en- 

 larged, and the walls of the capsules as withdrawn into, or 

 becoming continuous with, those of the cranium ; or if the 

 latter be conceived as disappearing, while the former take 

 their places, the general arrangement of the ethmoidal section 

 of the persistent " primordial cranium " of the ray will be 

 seen to be similar to that sclerotome in the bird's skull which 

 retains most of the primordial character. The development 

 of the imperfect maxillaries in contact with the lower aspect of 

 the slightly-ossified inferior wall of the combined capsular 

 and neural mass, and the formation of the ethmoido-frontals 

 in the perichondrium which covers its upper surface, would 

 reduce the entire arrangement to the type of the correspond- 

 ing parts in the bird. 



By a similar process, the sessile cartilaginous auditory 

 capsules of the cyclostome may be conceived to become buried 

 in the temporal portions of the cranial wall in the plagiostome, 

 while in the osseous fish, after the primordial cranium has 

 become enveloped in the bony plates, which are formed in its 

 substance and in its fibrous covering, the auditory capsules 

 pass into the cranial cavity, having been enclosed by the 

 neurapophyseal and meta-neurapophyseal bony pieces of their 

 own and neighbouring sclerotomes. 



