PACHYRHIZODUS. 127 



The premaxilla in the group of jaw-bones referred to Hypsodon leivesiensis by 

 Agassiz (op. cit., pi. xxv a, fig. 2) is much fractured, but is better preserved in 

 the originals of PL XXV, figs. 3, 3 a, 3 b, 4, 4 a. As shown in upper and lower 

 view (figs. 3, 4), its shape nearly resembles that of the premaxilla of P. basalis ; 

 but, as seen in front view (fig. 3 a), the tumid base of the large inner tooth (?'.) 

 extends to the inner edge of the bone, while its crown curves inwards, and the 

 small lateral marginal teeth (m.) are nearly upright in implantation. The total 

 number of marginal tooth-sockets is probably about nine. An empty tooth-socket 

 is shown at s. in fig. 4, while the partially absorbed bases of shed teeth are seen 

 at b., b. in the same specimen. The maxilla is not completely known, but its total 

 length must have been from three to four times as great as that of the premaxilla. 

 As already described by Agassiz, the anterior half of the bone is very thick, but it 

 becomes a comparatively thin, laterally-compressed lamina behind, where it curves 

 slightly downwards. When seen in direct outer view (fig. 5) the bone appears to 

 taper in front, but here there is a great thickening, which is produced forwards and 

 inwards as a stout process, well displayed from below in PL XXVI, fig. 5. The 

 teeth, which are badly drawn in the figure by Agassiz quoted above, are much 

 smaller than the marginal premaxillary teeth (cf. PL XXV, figs. 4 and 5). The 

 dental crowns are smooth and curve inwards. No supramaxilla has been 

 observed. 



The type specimen of P. gardneri is merely the symphysial end of the dentary 

 bone, crushed and broken in such a manner as to mislead its original describer to 

 suppose that the bone is crossed by a fused suture. The nearly complete right 

 dentary is shown from below and from the side in PL XXV, figs. 8, 8 a. Its oral 

 border (fig. 8 a) is upright, but the greater part of the lamina of the bone (fig. 8), 

 is in a nearly horizontal plane. It is almost smooth externally, only marked by a 

 feeble groove which trends gradually upwards and backwards from the middle of 

 the symphysis towards the oral border. The shape of its hinder end is best seen 

 in the original of PL XXV, fig. 7, in which the comparatively short articulo- 

 angular bone (art.) also occurs, though slightly displaced and a little imperfect 

 behind. The teeth of the dentary are fixed on a stout ledge or shelf of bone, 

 which is always conspicuous when viewed from within (PL XXV, fig. 6). Their 

 stout bony bases are flanked outside by a thin wall of bone, which is more or less 

 broken in the fossils ; and they are so much crowded that their externo-internal 

 diameter is often greater than their antero-posterior diameter. The dental crown 

 is invariably smooth and conical, with the sharp apex curved backwards and 

 inwards. The teeth are in a single series, nearly uniform in size, only diminishing 

 at each end. On the thickened symphysis there is an additional tooth within the 

 outer row of small teeth (fig. 8 a, i.). The total number of tooth-sockets in the 

 dentary cannot be more than thirty. 



Horizons and Localities. — Zone of Schloenbachia varians : Glynde, Sussex. 



