MAESUPIALIA. 799 



turity. It is broken off at the middle of the first molar, and does not display 



any mental foramen up to that point. A cingulum descends from the anterior 



and posterior cusps to the base of the median on the outer side This species 



is about the size of Domnina gradata, and resembles it in the larger anterior 



teeth. 



Measurements, 



M. 



Length of last three molars 0052 



Length of third true molar 0024 



Elevation of third true molar 0018 



Depth of ramus at third true molar 0023 



Two other fragments of mandibular rami support teeth which have 

 much the character of those of the present species as regards the form and 

 proportions of the anterior triangle. The posterior triangle, however, is 

 like that of some of the other species, with two low and little-developed 

 cusps. One of the specimens includes the anterior base of the coronoid 

 process, which does not rise abruptly as in the type. These specimens 

 cannot be referred t© the P. marginale, and they differ from the P. alternans 

 in the anterior groove between the inner cusps of the anterior triangle. 



Peratherium alternans Cope. 



Embasm alternans Cope. Synopsis New Vertehrata Colorado, 1873, p. 4. Annual Report U. S. (Jeol. 



Surv. Terrs., 1873 (1G74), p. 468. 



Plate LXII, ligs. 22-24. 



Our knowledge of this marsupial rests on the posterior part of the 

 right mandibular ramus supporting the last two molars of a single individual. 

 All these parts present characters at variance with the species already de- 

 scribed. The anterior triangle is much elevated above the heel, and 

 appears at first sight to have but two cusps. The heel is low and concave, 

 and has two low cusps, neither of which is crescentic. On these accounts 

 I formerly referred this species to a genus distinct from the P. fugax, which 

 I called Emhassis. I believe, however, that the anterior cusp of the molars 

 exists, although in a rudimental condition, and that present evidence does 

 not warrant generic separation. 



The last molar is distinctly smaller than the penultimate, and its heel 

 is not narrowed, as in the typical species of this genus, but is truncate and 

 supports two cusps. There is a cingulum on the outer side of the anterior 

 triangle of both molars. The ridges at the base of the coronoid process are 



