920 DK. R. BROOM ON THE 



organs of Jacobson, they were really homologous with the bones 

 that unite to form the " dumb-bell bone " of Ornithorliynchus ; 

 and that the mammalian vomer had its homologue in the so-called 

 " parasphenoid " of the lower forms. As a new name was neces- 

 sary for the reptilian "vomers," I proposed the name j9revo??ier. 



In the Bati'achia we find all three bones well developed, the 

 median true vomer or parasphenoid being especially large to 

 support the base of the skull. When in the earliest true Reptiles 

 the pterygoids came together, there was little need for the median 

 vomer and it became greatly reduced. In the Cotylosaurian 

 Diadectes the median vomer is still a fairly strong rod, but in the 

 later Ootylosaurians or primitive Diapsidans Pariotichus and 

 Procolophon the vomer is a very short pointed process. In most 

 later Diapsidans the vomer remains a small unimportant element. 

 It developes to a fair size in the Ophidia and becomes large and 

 much specialised in the Chelonia. The prevomers, on the other 

 hand, remain large in most Diapsidans, but where, as in the 

 Chelonians and Crocodilians, the organs of Jacobson become much 

 reduced or lost the prevomers likewise tend to disappear. 



In the mammal-like Reptiles the vomer shows great variations. 

 In the Therocephalians it is small as in the pidmitive Diapsidans, 

 but with the development of a secondary palate a new function 

 is given to it, and it becomes large. In the Anomodonts, though 

 the secondary palate is only imperfectly formed, the vomer is large 

 and extends well forward. In the Cynodonts, where the secondary 

 palate is complete, the vomer is very large and extends from the 

 basisphenoid to near the front of the snout. The front part of 

 the bone corresponds so exactly in its relations to the mammalian 

 vomer, that it is impossible to doubt that the bones are homologous. 

 On the other hand, if the anterior part of the bone were lost it is 

 probable that every one would agree, from the relations of the 

 back part, that it was the homologue of the reptilian so-called 

 " parasphenoid." In Mammals the vomer varies greatly in size. 

 It is relatively very large in the Cetacea, sometimes extending 

 from the basioccipital to the front of the rostrum, while in the 

 Rodentia it is often more or less rudimentary. 



The prevomers ai'e large in the Dinocephalians and Thero- 

 cephalians. In the Anomodonts they have completely disappeared. 

 In the Cynodonts, with the formation of the secondary palate 

 they are either greatly reduced [Goonphognathus) or quite absent 

 {Bauria). In Mammals the prevomers are usually absent, their 

 function as supports to Jacobson's cartilages being taken by the 

 palatine processes of the premaxilla?. In onl}^ two mammals are 

 they known for certain to occur as distinct bones, viz. Ornitho- 

 rhynchus and Miniopterus, and in both of these the pair of bones 

 fuse together to form a median bone before the animal is full- 

 grown. 



Some observations have recently been made which at first sight 

 appear to cast a little doubt on the homology of the parasphenoid 

 with the mammalian vomer. 



