Homology of the Mammalian Alisphehoid Bone. 115 



the even more mammal -like Anomodonts, such as Dicynodon and 

 Oudenodon, there is a columella cranii, but no alisphenoid. When 

 we come to examine the Cynodonts — those remarkable reptiles, so 

 very nearly related to the mammals as to be regarded by many as 

 the group ancestral to them — we iind a broad flattened bone, which 

 most anatomists would not hesitate to call the alisphenoid, and yet, 

 there can be little doubt, it is only the columella cranii of the anomo- 

 dont flattened out. 



Probably the reason why the columella cranii is generally held 

 to be a distinct structure from the alisphenoid, is that the two appear 

 to develope differently. The former is manifestly or apparently a 

 development from the pterygoid bar, and hence it is frequently called 

 the epipterygoid. It is thus a part of the first arch. The ali- 

 sphenoid is usually supposed, on the other hand, to be a part of the 

 cranial wall. 



In both mammals and birds, it is difficult satisfactorily to study 

 stages sufficiently early to show clearly the origin of the ali- 

 sphenoid. In the developing marsupial, however, the appearances 

 are very unlike that usually figured in higher mammals. In the 

 primordial skull of a Trichosiirus embryo, measuring 10 mm. in 

 greatest length, the appearances are very remarkable. At a little 

 distance outside of the trabeculae, and opposite the pituitary fossa, 

 are two short cartilaginous rods, one lying on each side. The long 

 axis of each is parallel to the cranial axis. There is no structural 

 connection between the rods and any of the cranial cartilages. It 

 would seem to serve as a support to the large Gasserian ganglion, 

 and is far removed from any part of the brain. No one would 

 readilv suspect it had anything to do with the alisphenoid, but 

 would readilv believe it to be homologous with the peculiarly deve- 

 loped epiptervgoid of the Chamseleons, being similar in appearance 

 and having similar relations. In the Trichosurus embryo at birth, 

 then measuring 14 mm., the little rod is found to have taken on a 

 remarkable development. Its inner side has united with the trabe- 

 cula, and from its outer side an upward process has developed 

 between the two main branches of the fifth nerve. So far it is all 

 cartilage, but soon it extends further upwards by the addition of a 

 membrane-bone ossification. The ossification is not a distinct bone, 

 but a continuation of the cartilage ossification beyond the limits of 

 the cartilage. I have shown elsewhere that a similar development is 

 found in the Marsupial scapula. The alisphenoid may thus be 

 regarded as a peculiar specialisation of the little cartilage which is 

 apparentlv the modified pterygoid arch. 



The palatine bone begins to ossify immediately in front of the 

 anterior end of the little rod, and the pterygoid as a splint bone on 

 the under side of the front part. The relations of the palatine and 

 ptervgoid bones in lizards to the cartilaginous arch are very similar. 



Taking into consideration the evidence afforded by comparative 

 anatomy, palaeontology and embryology, I think, we are justified 

 in assuming that the mammalian alisphenoid is homologous with 

 part of the cartilaginous pterygoid arch of the reptiles. 



