166 DB. E. BBOOM ON THE STRtTCTUEE AKD [Juiie 4, 



In Udenodon (text-fig. 10, p. 167) the whole beak has had a horny 

 covering, which in some species at least covers not only the 

 alveolar margin, but almost the whole of the facial surface of the 

 maxilla {mx.) ; and in probably in all the species the hoi'ny layer 

 was specially developed over the caniniform development of the 

 maxilla. 



The premaxilla in Udenodon and Bicynodon is better deve- 

 loped than in the majority of reptiles, having not only a M'ell- 

 developed facial portion passing up between the two nostrils, but 

 also a large palatal portion. The two premaxillae, as in the bird, 

 must have veiy early united into a single bone (cf. PI. XVII. 

 fig. 2), as in even comparatively young specimens there is no trace 

 of a suture between the two elements. As I have elsewhere 

 shown (6), the premaxilla forms almost the whole of the bony 

 palate, including by far the greater part of the median ridge, which 

 has almost invariably been regarded as the vomer. In the paper 

 referred to, a section through the posterior part of the hard 

 palate of Udenodon truncatus is figured, and it is there shown that 

 though the maxillae have internal plates which form a sort of 

 secondary palate, these are almost completely covered by the 

 great palatal development of the premaxilla. From the palatal 

 portion of the premaxilla there passes up internally a prominent 

 median ridge which runs from the facial portion of the premaxilla 

 backwards to articulate with the vomer, and to a considerable 

 extent divides the nasal cavities. 



The maxilla differs in shape greatly in different genera, and it 

 is highly probable that there is considerable difference in shape in 

 the two sexes of the same genus. In all, however, there is a 

 more or less well-marked caniniform development. In some 

 species a prominent external descending ridge gives the tusk-like 

 development a triangular shape ; in others the descending process 

 is almost flat. The maxilla resembles that of man in having a 

 large antrum maxillare. From the region of the antrum, which 

 may be regarded as the centre of the bone, the maxilla is 

 developed upwards, forming the greater part of the side of the 

 snout (c/. text-fig. 10, p. 165) and bounded by the nostril and the 

 nasal (na.), the prefrontal {jpr.f.) and the lachrymal {Ic.) inwards, 

 forming part of the secondary palate ; and backwards, meeting the 

 jugal, are the palatine and the pterygoid bones. The caniniform 

 ridge is continued back into the ridge formed by the anterior bar 

 of the pterygoid and forms the border of the palate. 



I can find no evidence of any ossified turbinal bones. 



The nasals {na.) are moderate-sized bones irregularly ti'iangular 

 in shape. They are joined to each other by a fairly long median 

 suture. Their anterior sides are formed by the premaxilla {p.mx.), 

 the nostrils, and the maxillas. The posterior and outer side of 

 each nasal is bounded by the frontal (/V.), the prefrontal (pr.f.), 

 and maxillary (ni.v.). 



The lachrymal (Ic.) is quite mammalian in structure. It forms 

 a considerable part of the anterior wall of the orbit — fitting in 



