PRONOMOTHERIUM 165 



In some respects this species seems to be more primitive than the others, for example, the 

 angle is not so deep and does not begin to descend so abruptly at a point so far forward as in the 

 other species. Also the ramus is deeper and not so thick and heavy. This may be, however, a female 

 of P. madisonium, but we cannot be sure of its exact affinities until more material of the same form 

 is collected. At present I believe it to be most closely allied to that species. 



Pronomotherium laticeps (Douglass) 1900 

 Fig. 118; PI. XXIV, figs. 1-3 



Original Reference: New species of Merycochaerus in Montana. Pt. I. Amer. Jour. Sci. (4), X, 

 pp. 428-438, 3 text figs. {Merycochaerus laticeps). 



Type Locality: In the lower Madison Valley, near New Chicago, Granite County, Montana. 



Geologic Horizon: Upper Miocene (Madison Valley). Clay bluffs near New Chicago. 



Type: Genoholotype, Cat. No. 796 CM., nearly complete skull with lower jaws and with some bone frag- 

 ments, collected by E. Douglass in the summer of 1899. 



Specific Characters: The skull is approximately a fifth smaller than that of P. altiramus. 

 It is low and, as seen from above, roughly triangular in outline, widest across the zygomatic arches 

 and narrower at either end. The zygomata are relatively weak and more U-shaped than in 

 P. altiramus, with the maximum expansion just in advance of the glenoid surface. The malar is 

 deep below the orbit and, with the maxillary, flares outward markedly above M". The nasal bones 

 are very short, directed forward and upward, and their anterior edge is about on a line across the 

 antorbital margins. They are nearly triangular in outline, convex fore and aft and transversely, and 

 pointed anteriorly. The premaxillaries are coalesced with the maxillaries to form a trough-shaped 

 depression back to the nasals, a structure seemingly developed in response to a more or less flexible 

 proboscis. The sutures of the lacrimal bone cannot be clearly defined, but it is apparent that this 

 bone had almost no pars facialis and is confined almost wholly within the orbit. The antorbital 

 fossa has become greatly enlarged by the deeply excavated maxillaries to form inferiorly a broad, 

 nearly horizontal shelf above the posterior premolars and anterior molars. The nearly flat frontals 

 are reduced anteroposteriorly by the intrusion of the nasals. The orbits are elevated, vertically oval 

 in outline, and relatively large. The temporal ridges tend to converge rapidly at first, then less 

 rapidly, and finally they unite in a plane above the glenoid surface to form a thin well-marked 

 sagittal crest, about 65 mm. in length. The supraoccipital crest has a decided overhang, with the 

 wings wide apart. 



The brain case is short fore and aft, slopes downward rather rapidly behind the frontals, and 

 is relatively small. The external auditory meatus is not well defined, but apparently sloped out- 

 ward and upward. The palate was wide and concave. The palatonarial border is broken away to 

 some extent but appears to be opposite the middle of M 3 . The basicranial axis is very steep. The 

 glenoid surface is anteroposteriorly convex and transversely broad. The postglenoid processes are 

 flat behind and moderately convex in front, narrow anteroposteriorly, and of moderate breadth. 

 The paroccipital processes are broken but probably were fairly long. They are broad transversely 

 but narrow anteroposteriorly, with a convex face posteroinwardly and a concave face anteroex- 

 teriorly. The bulla; are not swollen but are very deep. Their shape is that of a quarter cylinder 

 terminated by a cone. There is an extreme shortening of the basicranial area, so that all of the 

 elements of this region lie in a comparatively narrow transverse zone, or, to express it in another 

 way, the length of the tooth row is nearly twice the distance from M 3 to the occipital condyle. 

 Index: 0.80. 



Mandible: The rami are not coossified at the symphysis. Anteriorly they are transversely 

 convex and downwardly concave, with an inferoposterior slope greater than in P. altiramus. The 



