554 MAMMALIAN AND REPTILIAN VOMERINE BONES, 



bone is not the vomer of OrnithorliyncJms^ and must in future be 

 referred to b}^ a new name. The name — Prevomer — which I 

 originally suggested (7) seems the most appropriate. 



In the large majority of the reptilian orders the so-called 

 '' vomers " are undoubtedly homologous with the prevomers of the 

 lizard. This is the case in the Ophidia, Rhynchocephalia, 

 Plesiosauria, Ichthyosauria, Pelycosauria, Dinosauria, and Pareia- 

 sauria. In the Theriodontia and Anomodontia the bone which 

 has been referred to as the vomer is the true homologue of the 

 mammalian vomer, and this is almost certainl}'- also the case in 

 the Chelonia. 



On the "Paraspiienoid " and the Mammalian Vomer. 



While the pair of bones in the front of the palate in the 

 Amphibia and other forms were regarded as the homologues of 

 the mammalian vomer, it became necessar}^ to find another name 

 for the large median membrane bone that supports the basicranial 

 axis, and the name " paraspiienoid " proposed by Huxley has been 

 almost universally adopted. 



This bone is greatly developed in the Amphibia, where, as a 

 rule, it forms a very complete floor to the cartilaginous cranium, 

 extending from the basioccipital region to near the anterior part 

 of the palate. It is invariably a median bone, and is ossified 

 from a single centre. According to Parker, the "paraspiienoid" 

 first appears in the frog as " a parostosis beneath the intertrabe- 

 cular space." 



Among reptiles the " parasphenoid," though almost alwaj^s 

 present, is as a rale feebly developed. In the primitive reptile 

 Sphenodon its development has recentl}^ been carefull}' worked 

 out by Howes and Swinnerton (13), according to whom " the 

 paraspiienoid first appears in the form of a minute bony element 

 . lying freety beneath the antero-median border of the 

 post-pituitary plate." The}^ add that " with advancing develop- 

 ment it assumes an arrow-head shape." 



In the lizard, Parker (14) figures the first appearance of the 

 " parasphenoid " as a minute median ossification lying just in 

 front of the pituitary space. 



