OF THE PELYCOSAl'RIAN GEXUS DIMETRODOX. 23 



The rorners f : Sutton ('84) and Broom ('02) have demonstrated that the bones 

 known as vomers in the. fishes, amphibians and reptiles are not homologous with the 

 bone known as vomer in the mammals, but they are separate ossifications of the 

 palatine region of the skull. 



It is impossible to reproduce the argument of Sutton's paper because of its length, 

 but the main points made are as follows : He first shows that the parasj)henoid of the 

 adult Pike and the vomer of the human fu3tus at birth have essentially the same 

 relations, and that in an earlier stage of the human fcetus, before the roof of the mouth 

 has closed, all the resemblance between the positions of the two bones is even more 

 striking. He shows that in the history of the development of reptiles from amphibians 

 the increased ossification of the basi-cranial bones does away with the need of a well 

 developed parasphenoid bone to support the floor of the brain case. He then demon- 

 strates the complex origin of the maxillary bone in the mammals and comes to the 

 following conclusions : 



'' It is now evident that for morphological purposes the superior maxillary 

 consists of four distinct portions — 



" (a) The premaxillary region in relation with the ethmo-vomerine cartilage and 

 the naso-palatine nerve. 



"{b) A prepalatine portion forming a platform for the support of the anterior end 

 of the vomer. 



" (c) A maxillary center situate to the inner side of the superior maxillary division 

 of the fifth nerve. 



"((/) The malar piece lying outside this nerve and sup{K)rting the maxillary bone." 

 He concludes that the prepalatine centers are the homologues of the vomere of the 

 amphibians because — 



1. They are membrane-formed bones. 



2. The bone in each case underlies the anterior end of the vomer and parasphe- 

 noid, respectively. 



3. Although in tlie Pike the so-called vomer is median and single, nevertheless in 

 Lepidosteus, Rana, jMenobranchus and many other {reptiles) forms, the bones so called 

 are double. 



4. In their relation to the premaxilhe and palate bones they fulfill the required 

 anatomical conditions. 



In his work on the origin of the mammalian vomer Broom ('03), after a careful 

 and full discussion of the relations of the bones, gives the following conclusion, p. 354 : 

 " In the large majority of the reptilian orders the so-called " vomers " are undoubtedly 

 homologous with the prevomers of tiie lizard. This is the case in the Ophidia, Rhyn- 



