370 Transactions of the South African Philosophical Society. 



what is probably its primitive condition as a nasal floor bone. In 

 Therocephalians and Cynodonts the relations of the bone have not 

 been satisfactorily made out, but so far as is known it forms, as in 

 Sphenodon, the floor of the anterior part of the nasal cavity and 

 probably also protects the organ of Jacobson. 



In mammals the organ of Jacobson is usually relatively small and 

 well enough protected by its cartilage at the base of the septum. 

 The nasal floor is also well protected by the secondary palate. There 

 would consequently seem to be no use for a septomaxillary, and as 

 might be expected it is almost invariably absent. About ten years 

 ago, however, when working at the mammalian organ I came across 

 a hitherto undiscovered bone in the floor of the nose of the Arma- 

 dillo, Dasypus villosus. Though a description of the bone was pub- 

 lished at the time, I was unable to come to any conclusion as to its 

 homologies, and it was only when recently working at the organ of 

 Jacobson in Sphenodon that I recognised that the septomaxillary 

 of that reptile is essentially similar in its relations to the nasal floor 

 bone of the armadillo. I therefore consider that the small bone in 

 the nose of Dasypus is a true septomaxillary which has for some 

 reason been retained from the reptilian ancestor. In Dasypus 

 villosus there is a pair of crescentic bones, but in Dasypus minutus- 

 the bones are anchylosed together. 



Prevomer. 



Some years ago I endeavoured to show that the paired bones in the- 

 front of the palate of the lizard which are usually called " vomers," 

 are not homologous with the mammalian vomer. The vomer of the 

 mammal is a median bone which forms as a splint on the basicranial 

 axis. It may extend from the front of the axis to the basioccipital as in 

 many Cetaceans, or it may be quite rudimentary, as in many rodents. 

 In Echidna it is situated far back and does not appear in the anterior 

 nasal region. In its relations and development it agrees exactly with 

 the reptilian bone called " parasphenoid," as was pointed out first by 

 Bland Sutton. The so-called "vomers" of the reptile are paired 

 bones which have nothing to do with the basicranial axis, but 

 develop as splints on the paraseptal cartilages, and principally 

 serve as a protection to the organs of Jacobson. In Ornithorhynchus- 

 the paraseptal cartilages are protected by a pair of bones exactly as 

 iu lizards. These become anchylosed to form the " dumb-bell shaped 

 bone " of the adult, but there can be little doubt that they are 

 homologous with the so-called " vomers " of reptiles. As a new 

 name was necessary for them I proposed to call them " prevomers." 



