Presence of a Pair of Distinct Prevomers in Titanosuchus. 



145 



palatal region of most reptiles and batrachians are together homolo- 

 gous with the vomer of the mammals. Till 1884 I am not aware of 

 this opinion having ever been called in question. In that year, how- 

 ever, Bland- Sutton " advanced the opinion that the mammalian 

 vomer is homologous with the so-called " parasphenoid " of batra- 

 chians. In 1885 W. K. Parker f in dealing with the development of 

 the Insectivore skull, makes use of expressions which seem to indicate 

 that doubt had arisen in his mind as to whether the so-called 

 " vomers " of lizards and snakes are really true vomers. 



In 18951: I endeavoured to show that the so-called " vomers" of 

 lizards and snakes are really homologous with the dumli-bell bone of 

 Ornithorlujnckus, both being developed as a pair of splints to 

 Jacobson's cartilages. 



Since then numerous facts have been brought forward all pointing 

 to the conclusion that the mammalian vomer is the homologue of 

 the median bone which supports the base of the cranium in batra- 

 chians and most reptiles, and which was named by Huxley the 

 "parasphenoid," and that the bones which had been called " vomers" 

 in the lower forms are entirely different in their morphological 

 situation, and that they are represented in mammals by the dumb- 

 bell bone of Ornithorhyiichus, and by a little median bone in the nose 

 of Miniopterus. For these anterior elements I proposed in 1895 the 

 name of prevomers. The whole question has recently been fully 

 treated in a paper read before the Linnean Society, N.S.W. (Oct., 

 1902). § 



The most conclusive evidence in favour of the truth of this position 

 is that derived from palaeontology. The mammals we know are 

 derived from the Theriodonts : the Theriodonts probably from a 

 more primitive Theriodont type, such as Titanosuckns : these latter 

 from a form something like Pareiasaurns : the Pareiasaurus prob- 

 ably from a more primitive type allied to Procolophon : which latter 

 is not very far removed from the Microsaurian Labyrinthodonts. 



If we look at the evolution of the prevomers in each type it becomes 

 at once manifest that the old view is erroneous. 



In the Labyrinthodonts, as in other batrachians, in the front of the 



* J. Blaud-Sutton, " Obseiv. on the Parasphenoid, the Vomer, and the Palate 

 pterygoid arcade," P.Z.S., 1884. 



f W. K. Parker, " On the Structure and Development of the Skull in the Mam- 

 malia," Phil. Trans., 1885. 



+ K. Broom, " On the Homology of the Palatine Process in the Mammalian Pre- 

 maxillary," P.L.S., N.S.W. , June, 1895. 



§ R. Broom, " On the Mammalian and Reptilian Vomerine Bones," P.L.S. 

 N.S.W., Oct., 1902. 



