MORRISON MAMMALS: PANTOTHERIA 73 



YPM- I/&Z2. 



Fig. 28. Herpetairus arcmtus. Right upijer molars, crown view, type. 

 Fourteen times natural size. 



slightly outward. From the varying widths of the molars it hence results that the outer 

 contour of the series is more strongly arcuate (hence the specific name, which would be 

 equally appropriate for any of the upper jaws here described). It does not follow, as 

 implied by Marsh, that the lower molars were in a similarly arcuate series. Indeed the 

 fact that the lower molars were not arcuate but varied in size and occluded with upper 

 teeth of even more variable size is the apparent mechanical concomitant (whether cause 

 or not) of this arched arrangement. 



From these varying proportions and details of arrangement it further follows 

 that the angle between the anterior and outer edges of the molars varies. On M*"' this 

 angle is over 90°, on M* it is about 90°, and on M'"^ it is progressively less than 90°. 



Each molar is implanted by three roots, two external and one internal, but the two 

 external roots divide farther and farther from the crown on the more posterior teeth 

 and on M^ they appear to be confluent, although there are two pulp cavities and the 

 confluent roots are separated by a vertical groove externally. 



The internal angle is occupied by a single cusp, the chief apex of the crown, which 

 is high and slender. Like the pr'^ of the lower molars, this cusp is made to seem even 

 more lofty by the fact that the alveolar border is lower on this side of the crown. This 

 internal cusp is crescentic, sending ridges outward along the anterior and posterior 

 borders of the crown. There is an apparent torsion of the tooth which causes this inner 

 portion to point slightly posteriorly. 



The outer border of the crown has three cusps. The largest, slightly smaller than 

 the internal cusp, is central and is directly opposite the latter. It is somewhat com- 

 pressed transversely and sends a sharp low ridge backward to the posteroexternal cusp, 

 and another forward to the anteroexternal one. The latter is anterior and somewhat 

 external to the central cusp. From its apex a distinct and often sharp crest or cingulum 

 runs upward and backward on the outer face of the crown, to end almost directly above 

 the tip of the central cusp. The posteroexternal cusp is of about the same size and is sub- 

 crescentic. One wing is formed by the crest running to the centroexternal cusp and one 

 by the crest along the posterior edge of the molar. Along this posterior edge, inter- 

 mediate between the internal and posteroexternal cusps, is another cuspule which is 

 also subcrescentic. One wing is shai-p and is that running along the posterior edge to 

 the posteroexternal cusp, the other is rounded and runs to the internal side of the base 



