THE SKELETON OF THE VERTEBRATE HEAD. 171 



The external nostrils of the lacertian, ophidian, and am- 

 phibian, are situated, as in the bird, between the vomerine 

 and ethmoidal sclerotomes ; the intermaxillaries being closed 

 in front and below. The so-called nasal fossae in these verte- 

 brate forms are also, as in the bird, merely olfactory chambers, 

 occupying the neural space of the ethmoidal sclerotome. The 

 posterior nares, too, open as in the bird, between the eth- 

 moidal and pre-sphenoidal sclerotomes, but with the follow- 

 ing subordinate differences : In the lizard they are separated 

 by the anterior extremities of the ento-pterygoids, and are 

 bounded behind by the maxillary processes of these bones, 

 and externally by the rnaxillaries themselves. In the ophidian 

 they are separated by the free margin of the ethmoidal cata- 

 centric plates ; anteriorly by the posterior margins of the eth- 

 moidal neurapophyses, externally by the anterior projecting 

 portions of the ento-pterygoids, and behind by the pre- 

 sphenoidal attachments of the latter. In the frog they open 

 between the ethmoidal neurapophyses ("vomer"), the ento- 

 pterygoids (" palatals "), and the maxillaries. 



We again approach the mammalian type of nasal fossse, 

 through the tortoises, turtles, and crocodiles. 



It has been already stated that the anterior nostrils of the 

 Chelonian appear to possess more of the ornithic than mam- 

 malian conformation. The primordial cartilaginous lining of 

 the olfactory fossse projects in some turtles through the an- 

 terior nasal opening of the cranium in the form of a double 

 proboscis. The posterior nares in the tortoises are separated 

 by the combined ento-pterygoids (upper and back part of the 

 " vomer"), and are bounded by the maxillaries and the pala- 

 tines, the latter remaining open or ununited across the vault 

 of the palate. In the turtles, the vault of the palate and the 

 posterior nares present more of the mammalian aspect, 

 although still formed essentially on the type of the corre- 

 sponding parts in the bird. This is effected by the ethmoidal 



