MORRISON MAMMALS: MULTITUBERCULATA 19 



Ma is a little broader than M,, but it also has six cusps in two rows of three each. 

 These cusps all appear to be of about the same size on this tooth. Both molars are im- 

 planted by two stout roots, each slightly compressed anteroposterior! y. The roots are 

 developed vertically to the alveolar border, which is oblique, while the crowns are hori- 

 zontal. Consequently the roots are obliquely affixed to the crowns. 



Mandible 



The mandible turns inward quite sharply anteriorly at the symphysis. The sym- 

 physial surface is short, not quite extending back to a vertical line anterior to Pi. 

 There is a single mental foramen beneath the diastema. When the jaw is oriented with 

 the grinding surface of the molars horizontal, the mandible is oblique, the anterior 

 portion much lower than the posterior. Beneath Po there starts a slight ridge, much 

 emphasized by Marsh, which curves backward on the external face of the mandible and 

 passes into the anterior border of the coronoid. The latter arises external to Mo and is 

 rather weak and short." A number of misapprehensions have arisen with regard to the 

 posterior part of the mandible. Marsh represented it (1887, PI. VIII, fig. 4) as having 

 a distinct inflected angle, and he considered the distinctness of the angle to be in con- 

 trast with Plagiaulax. Reexamination shows that the original is slightly crushed and 

 that careful reconstruction gives a condition exactly like that in other multitubercu- 

 lates. There is no proper angular process, only a well developed pterygoid crest run- 

 ning forward from the articular condyle. On the opposite side (outside) of the jaw is 

 a somewhat similar but less prominent, efflected masseteric crest. The dental foramen 

 is at the anterior end of the pterygoid fossa, just posterior to the roots of Mo. The 

 condyle is not separated by a restriction from the rest of the mandible. The articular 

 surface looks upward and backward. It has a broad, gently convex superior surface, 

 for grinding, and a short, more sharply convex, posterior area, used in opening and 

 closing the mouth. 



Measurements 



The unusual orientation of these jaws makes the selection of useful standard 

 coordinates difficult. The following tooth measurements are maximum lengths taken 

 along the inclined alveolar border. All measurements in this memoir are in milli- 

 meters. 



« Marsh's original figure (see 1887, PI. VIII, fig. i) restored the posterior part of the mandible 

 before it was adequately known and is incorrect in many particulars, as may be seen by comparing with 

 his PI. VIII, fig. 4 and with the present figures. In PI. VIII, fig. 4, the angle is incorrectly shown and 

 the figure is inadvertently labeled "x3," whereas it is actually X4. 



