798 THE WHITE ElVER FAUNA. 



stronger, and the fossa extends further forwards than in any of the various 

 specimens of P. fugax which display this region. There is no internal 

 pterygoid fossa as in P. alternans. 



Measurements. 



Length of last three molars .•'. 0660 



Length of second true molar 0020 



Elevation of second true molar 0022 



From the same locality and horizon as the preceding species. 

 Peratheeium marginale Cope. 



Berpetotherium marginale Cope. Synopsis New Vertebrata, Colorado, 1873, p. 6. EmbassU marginalit 

 Cope, Ann. Report U. S. G. S. Terrs., 1873 (1874), p. 468. 



Plate LXII, figs. 19-21. 



This species has a more slender mandibular ramus than the P. fugax, 

 but teeth of the same size, thus having the proportions of the P. tricuspis. 

 The anterior triangle is more elevated than the posterior, and is not fissured 

 on the inner side so as to distinguish the anterior and middle cusps, except 

 at an elevated point. From the fissure between them a shallow groove 

 descends inwards on the front of the tooth. The inner principal cusp is 

 thus as large as the outer, and the triangle is transverse, or much broader 

 than long. The species is also peculiar in the constitution of the posterior 

 triangle or heel, which supports two elevated acute cones, which are some- 

 times curved forwards. The middle of the heel is concave. I at one time 

 thought that the P. marginale was, with the P. alternans, worthy of generic 

 separation, but so many intermediate conditions connect this form with that 

 seen in P. fugax, that I no longer maintain this opinion. 



There are four molars in the typical specimen, a part of the first being 

 broken away. The last true molar is I'ather smaller than the others, and 

 relatively smaller than in P. fugax, and is not narrowed posteriorly, sup- 

 porting two opposite well-developed cusps instead of the thi-ee weak ones 

 of the other species. It is crowded close to the base of the coronoid process, 

 instead of standing well in front of it as in P. huntii and P. fugax, which 

 gives the appearance of not being fully protruded, although its cusps have 

 the same elevation as those of the other molars. The inferior border of the 

 ramus rises strongly at this point, much more so than in the two species 

 named, showing that the crowding of the fourth molar is not due to imma- 



