SKULLS OF CYXODONT REPTILES. 919 



traced round to the top of the snout. Were these tracts ossified 

 we woukl have a coiidition practically similar to that of the 

 Cynodont. 



Sejytomiaxillary . 



Kitchen Parker many years ago recognised this as a distinct 

 membrane-bone in the Lizards and Snakes, but it is only recently 

 that much attention has been paid to it, chiefly as the result of the 

 work of one or two pal?eontoIogists in Europe and America. The 

 bone is not known in Labyrinthodonts and probably does not 

 occur in the Amphibia (the supposed septomaxillary of Xenopits 

 being probably not homologous). We find it, however, in the very 

 earliest true reptiles, and we can trace it on the one hand through 

 numerous members of the Diapsida, and on the other through 

 most of the mammal-like Reptiles on to Mammals. 



A septomaxillary has been found in Pareiasaurus, Pariotichns, 

 and Procolophoii. In these primitive genera it is mainly within 

 the nostrils, and probably fulfils its main function as a roof to 

 Jacobson's organ. 



When we come to the mammal-like Reptiles, we find it in the 

 Pelycosaurs still mainly within the nostril. In the Dinocephalians 

 {Delpliinognalhus, Tcqnnocephcdus) it comes partly on to the face. 

 In the Dromasaurians (Galejms), it forms a very appreciable 

 portion of the facial wall ; and in the Therocephalians {Scylaco- 

 saurus, Alojyosaurus), it also appears pretty largely on the face. 

 In the Anomodonts it is absent, probably because they had lost 

 their organ of Jacobson, as would appear from the loss of the 

 pre vomer. 



In the Cynodonts, the septomaxillary is always present. In 

 the lower types it appears on the face, but in the higher forms it 

 is almost entirely inside the nosti'il. 



Among Mammals a septomaxillar}- is known only in some of 

 the lower forms. In Tritylodon it appears on the face between 

 the nasal and i3remaxillary, in much the same way as in Nytho- 

 saurus. In Ornithorhynchus and Echidna it would appear from 

 the researches of Gaupp that what used to be regarded as the 

 upper part of the premaxillary is really the septomaxillary. If 

 this be so, as seems pretty certain, then the Monotremes have the 

 septomaxillaries better developed than in the Cynodonts. 



The only higher mammal in which there is a bone to be regarded 

 as probably the septomaxillar}' is Dasypus. Here a small bone, 

 which I described in 1897 as the " nasal-floor bone " and suggested 

 might be homologous with the upper part of the premaxillary in 

 the Monotreme, is probably to be regarded as a rudimentary septo- 

 maxillary. 



Yoiner and Prevomers. 



In 1895, and more fully in 1902, I showed that there was 



reason to believe that the so-called reptilian " vomers " were not 



homologous with the mammalian vomer, but that being formed as 



splints to the pai-aseptal cartilages in close association with the 



63* 



