184 DE. R. BROOM ON THE STEUCTUEE AND [ Juiie 4, 



palate the anterior paired element — the so-called " vomer " — being 

 no longer required as a bony floor for the nasal cartilages, has become 

 reduced, and is only retained to give support to the cartilages of 

 Jacobson's organ. This is the condition we find in Gomphognathus 

 (12). The " parasphenoid," having now a new function — that 

 of forming a support for the secondary palate — becomes greatly 

 developed as a median plate, which is unquestionably the homo- 

 logue of the mammalian vomer. From this it follows that the 

 element called Parasphenoid in the Reptiles generally ought to be 

 called the Vomer ; while the anterior paired element, usually 

 regarded as the " vomer," but which is the homologue of the 

 dumbbell-shaped bone in OniitJiorJiynchus, may be called the 

 " anterior vomer," or by the name I have elsewhere (13) proposed 

 for convenience, the Prevomer. 



In Udenodon the true vomer is greatly developed, but the 

 anterior vomer or prevomer is lost — no doubt owing to the great 

 palatal development of the premaxillary. The vomer in the 

 Anomodonts agrees with the mammalian vomer even more closely 

 than does the vomer in the higher Theriodonts. 



In the Chelonians, probably as the result of a parallel develop- 

 ment, the palate bears considerable resemblance to that in the 

 Anomodonts, and it even appears that the median vomer is a true 

 vomer, as in Dicynodon and its allies. 



The lower jaw of Udenodon differs greatly in appearance from 

 that in the Theriodonts, owing to its being toothless and to the 

 absence of a well-developed coronoid process. As the elements 

 are apparently the same, the difference is probably due largely to a 

 degeneration in the Anomodont jaw, similar to that which has 

 taken place in the toothless mandibles of such mammals as 

 Echidna or the Whales. 



The vertebrae appear to be essentially similar in structure in the 

 Anomodont and in the Theriodont ; but in the Anomodont the 

 type is somewhat more primitive, in that the ribs of the lower 

 trunk vertebras are not specialized as they are in the higher 

 Theriodont at least. 



The shoulder-girdle in Udenodon belongs to the type which is 

 found to persist with little variation from the lower forms such as 

 Pareiasaurus up to the Monotremes. The cartilaginous elements 

 consist of a scapula, a coracoid, and a distinct precoracoid, while 

 the membrane-bone elements are a clavicle, a supra-clavicle or 

 cleithrum, and a median interclavicle. In the Anomodonts the 

 coracoid and the precoracoid are extremely like those in the 

 Theriodonts, both the higher and the lower ; but, curiously enough, 

 the scapula agrees with that in the higher Theriodonts such 

 as Cynognathus, and differs from that in the lower Therio- 

 donts,^ e. g. Ictidosuclius, in having a moderately well-developed 

 acromion. 



The humerus closely resembles that in the Theriodonts, but 

 whereas in the latter, as in the typical terrestrial mammals, the 

 humerus is elongated, in the Anomodonts, as in the Monotremes, 



