THE SKELETON OF THE VERTEBRATE HEAD. 117 



margin of the nostril, as the transverse limb of the crutch-like 

 inferior actinapophysis of the ruminant supports the outer 

 margin of the orifice. 



TJie Rudimentary Rliinal Sclerotome in the Crocodiles. In 

 the crocodiles, as in the mammalia, the vomerine sclerotome 

 is traversed by the nasal fossae, which open therefore in front, 

 instead of behind it, as in the other reptiles and in birds. It 

 is evident therefore, that if the crocodiles do not possess, like 

 the mammalia, a rhinal sclerotome, their external nostrils 

 must present an exceptional arrangement ; for, instead of 

 being metasomatomic, they must be terminal. I find, how- 

 ever, that in the alligator the hoods which extend from the 

 anterior inner margins and septum of the osseous external 

 nostrils consist of dense fibrous tissue, covered by the muscles 

 which act upon them. This double fibro-muscular hood is so 

 arranged on each side as to have the oblique slit-like nostrils 

 situated between their outer margins and the intermaxillary 

 edges. If a plate of cartilage were developed in the margin 

 of each hood, the whole arrangement would occupy the posi- 

 tion and exhibit the relations of an ali-nasal cartilage a 

 rhinal hsemapophysis, or neurapophysis, as in the elephant. 



VOMERINE SCLEROTOME. In Mammals. In the mammal 

 the vomerine is a perfect catacentric sclerotome. The nasal 

 bones are its neural elements, as they occasionally run together, 

 and are evidently, as has been generally admitted, serially 

 homologous with the frontals and parietals ; they must be 

 viewed as metaneurapophyses, the neurapophyses being absent 

 in the absence of a nervous centre. The intermaxillaries meet- 

 ing below form the haemal arch, and the centrum consists of 

 the vomer, with a corresponding portion of the cartilaginous 

 nasal septum. 



The vomer is a bone peculiarly mammalian. It may be 

 said to make its appearance as a developed element, along 

 with the completed nasal fossae. But its development in the 



