HYPSODON ETC. FROM BRITISH CRETACEOUS STRATA. 513 



are very irregular, and it is not easy, especially in a side view such 

 as figure 2, to determine whether the small teeth are fully grown 

 ones, or are large teeth only partly grown. It becomes necessary 

 therefore to consider the relative sizes of the alveoli and their teeth 

 in order to know whether the latter are fully developed. The im- 

 portance of knowing this will be obvious when it is remembered 

 that the relative sizes and number of the teeth have been regarded 

 as important specific characters. 



The maxilla has two articular surfaces at its upper and front 

 part ; the anterior of these is laterally compressed and placed on the 

 inner side of the process of the praemaxilla, with a deep depression 

 intervening. The posterior articular surface lies immediately behind 

 and above the praemaxilla. It is this one which articulates with the 

 palatine bone. The upper margin of the maxilla is strongly grooved, 

 probably on account of its articulation with the suborbital bones. 

 Towards its posterior end it is gradually reduced in thickness ; 

 both maxillae, however, are slightly broken in this region. The 

 dentary border is concave at its anterior part and nearly straight 

 posteriorly. The anterior nine or ten alveoli are small (about ■£% inch 

 diameter) ; these are succeeded by ten or twelve larger ones (about 

 ginch diameter); behind these there are indications of perhaps eighteen 

 or twenty small ones. Most of the teeth are broken off; but those 

 which remain are somewhat curved inwards — a character also in- 

 dicated in Cope's figures of P. lestrio, but not alluded to in the text. 

 The right maxilla has a small portion of another bone attached to 

 the upper edge of its posterior extremity, which may be the repre- 

 sentative of the jugal bone. 



Comparing these maxillae and praemaxilla) with those of P. lestrio 

 (I. c. pi. -±2, fig. 1), it will be seen that the general contour is not 

 the same. In the present specimen the praemaxilla is directed 

 more obliquely backwards, its anterior process is more prominent 

 and placed further forward, leaving a greater space between it 

 and the posterior maxillary articular surface. The groove and 

 notch between the maxilla and praemaxilla are not seen in the 

 figures of P. lestrio. The larger teeth of the maxilla are placed 

 further backwards in our specimen ; and consequently the con- 

 vexity of this margin is further from the praemaxilla. 



Mandible. — The dentary portions of both rami are preserved, 

 and, as in other species of this genus, are remarkable for their 

 great vertical depth and compressed form. A small portion of 

 the articular bone of the left side is preserved (fig. 1, art*) ; but that 

 of the right is altogether wanting. The outer surface of the dentary 

 has a deep groove towards the lower margin. The anterior portion 

 is swollen, especially its upper part, to accommodate the large ante- 

 rior teeth. There are depressions upon this surface, which probably 

 indicate tho extent to which the articular bone penetrated the den- 

 tary portion. A few vascular channels may be traced radiating 

 from a point close to the anterior end. The lower margins are 

 mostly broken away, on account of their being very thin below the 

 * This figure has been reversed. 



