BY R. BROOxM. 555 



In the snake the " parasphenoid " is moderately well developed, 

 and extends far forwards. In its earliest stage of development 

 it is described by Parker (15) as "a lx)ng styloid tract of granular 

 tissue . . . wedged between the trabeculag " and reaching 

 *' the pituitary body by its broad hinder end." 



It will thus be seen that in both amphibians and reptiles the 

 '' parasphenoid " is a median ossification which develops in the 

 neighbourhood of the pituitary space — occasionally arising behind 

 the space as in Sjyhenodon, and at other times, as in the snake, 

 having its entire development in front of the space. 



Now, the mammalian vomer is developed as a median splint 

 bone along that part of the basicranial axis which in later 

 development becomes the presphenoid and mesethmoid regions; 

 and as this is exactl}^ the region in which the median splint bone 

 of the snake is developed, we are forced to conclude that the 

 "parasphenoid" of the snake is really the homologue of the 

 mammalian vomer. In the reptiles, no doubt the exact position 

 of the median bone varies considerably; but so in mammals does 

 the vomer vary greatly as regards its anterior and posterior 

 developments. In most mammals the vomer extends far forwards, 

 and in the Cetaceans extends practically to the end of the long 

 rostrum; but in Echidna, on the other hand, the anterior extremity 

 of the vomer is found in the posterior nasal region; and while in 

 many Rodents we find the vomer rudimentary, and posteriorly not 

 even reaching the presphenoid, in some Cetaceans the vomer 

 extends back even to the basioccipital. 



The Phylogeny of the Vomerine Bones. 



Having shown that the mammalian vomer has its homologue 

 in the " parasphenoid " of the reptiles and amphibians, and that 

 the so-called " vomers " of these lower forms are really homologous 

 to the dumbbell bone of Ornithorhynchus, it is now only necessary 

 to consider briefl}^ the steps b}' which the vomerine bones of the 

 higher forms have been derived from those of their Labyrinthodont 

 ancestors. 



