556 MAMMALIAN AND REPTILIAN VOMERINE BONES, 



In the extinct Labyrinthodonts, as in their degenerate descen- 

 dants, the Amphibia of to-day, we find the basicranial axis 

 supported by a powerful median vomer (parasphenoid), and in 

 front we have a pair of prev^omers supporting the nasal capsules. 



In the most primitive reptilian order with which we are 

 acquainted — the Cotylosauria — we find that the palate differs 

 from the ancestral Labyrinthodont condition mainly in the greatly 

 increased development of the pterygoids, with a corresponding 

 decrease in the size of the median vomer. Howes has already 

 recently suggested that the reduction of the " parasphenoid " in 

 the ascending reptilian series has been brought about by the 

 increased development of the pterygoids; and it seems moderately 

 certain that this is the true explanation. It is more difficult to 

 assign a cause for the increased development of the pterygoids. 

 But it seems probable that as the descendants of the Labyrintho- 

 donts took to terrestrial habits a stronger palate than could be 

 formed by the median vomer became advantageous, and was 

 formed by the pterygoids, which, besides becoming greatly 

 increased in size, became dentigerous. 



In the majority of more recent reptiles, we have a modification 

 of the condition found in Sphenodon where there is a small 

 median vomer and two moderately large prevomers. 



In the Theriodonts, however, an interesting modification arises. 

 With the formation of a secondary palate a new function is given 

 to the median vomer, which becomes again well developed as a 

 support to the new palate. The prevomers, on the other hand, 

 being now no longer required to protect the nasal capsules, become 

 greatly reduced in size, and are retained only as supports to the 

 cartilages of Jacobson, 



In the Dicynodonts the true vomer is greatly developed, while 

 the prevomers are completely lost owing to the enormous develop- 

 ment of the premaxillaries. 



In the Chelonians we have a condition somewhat resembling 

 that of the Dicynodonts, and probably brought about by a 

 parallel development. The median vomer is developed as a 

 support to the palate, and the prevomers are lost. 



