250 THE MERYCOIDODONTID/E 



so that a definite statement cannot be made. These bulla? are enormously inflated and in shape are a 

 slightly compressed oval, with the long diameter anteroposterior. They fill the entire space between 

 the postglenoid process and the basicranial axis. The postglenoid process is rather small and is 

 entirely covered posteriorly by the tympanic bone. Its internal border abuts against the bulla. The 

 paroccipital process is in contact with the bulla anterointernally, and is moderately wide in an antero- 

 external diameter but compressed in the other. The tip has been lost, but the inferior part seems to 

 have been rather square in section and to have extended below the inferior part of the bulla. 

 Index: 0.71. 



Mandible: The symphysis is coossified and the suture obliterated. It is short with a very 

 steep anterior face, except at the alveolar margin, where it is everted anteriorly. The horizontal 

 ramus decreases regularly in depth forward, and the angle of the mandible was probably full and 

 round. 



Foramina: The infraorbital is above the anterior part of P 4 . In the type this foramen is 

 double, but as this condition is seldom met with in other members of the family, I do not consider 

 it normal. The supraorbital (frontal) foramina are equidistant between the median suture and the 

 supraorbital border. 



Dentition: Superior: The incisors are small. The canine is small and separated from P 1 by a 

 diastema equal to its diameter. In section it is semicircular, with the truncated face postero- 

 internal. P 1 is single-rooted, trenchant, and triangular in section at the base of the crown, the faces 

 being anterior, external, and posterointernal, with a weak internal basal cingulum. P 2 is double- 

 rooted, with one main cutting edge and with a strong internal basal cingulum. P 3 has an external 

 crescent and a rudimental internal one, not united in front. Both P 2 and P 3 are obliquely quadrate, 

 that is, they are a little wider posteriorly than anteriorly, because of the presence of a half crescent 

 internally, the posterior horn of which is attached to the external wall, while the anterior is free. 

 The external faces of these two teeth are slightly convex. P 4 has well-developed inner and outer 

 crescents only and a slightly concave outer face. 



The molars increase moderately in size from M 1 to M 3 . The parastyles and mesostyles also 

 increase progressively in prominence in the same order. The metastyle on M 3 is quite weak and 

 does not form a heel. M 1, 2 are rather quadratic, while M 3 is prismatic, with a reduction of the 

 hypocone. 



Inferior: The incisors are small, subcylindrical, crowded between the canines, and forwardly 

 directed. The canine is much larger, with a cylindrical root and a transverse crown. Pi is some- 

 what larger, and about the same form as C, from which it is separated by only a slight divergence of 

 crowns. There is no diastema between Pi and P 2 , both of which are longer than wide. P 2 is two- 

 rooted, with a compressed triangular crown and with a median ridge on the internal side. P 3 and 

 P 4 have the anterior parts trenchant. The anterior three premolars have their long axes directed 

 anterointernally. The molars are prismatic and increase in length posteriorly. Mj and M 2 are 

 nearly subequal and together are longer than M 3 . The latter does not quite equal in length that of 

 the last three premolars combined. M 3 is placed somewhat obliquely and has a large metastylid. 



Cope described the milk dentition as follows (1884B, p. 551): 



The third superior temporary molar has two pairs of crescents. The anterior pair are, however, not so well 

 developed as the posterior pair and the two valleys are soon obliterated by wear. The crescents are equal in the 

 fourth temporary molar. The fourth permanent premolar is protruded at least as soon as the third true molar, 

 sooner than the posterior column of the latter. In this it differs from the Oreodon culbertsonii, where the last 

 true molar is protruded first, and is a cotemporary of both the third and fourth deciduous molars; and the 

 O. gracilis, where the last true molar is a cotemporary of the third deciduous. 



In the inferior temporary dentition, the lobes of the last molar are subequal, the posterior one being a little 

 the larger. The protrusion of the last true molar is also probably delayed until the shedding of the deciduous 

 series, as in the superior series; but my specimens are either very young or fully adult, and therefore I cannot 

 demonstrate this point as fully as in the case of the superior series. 



