340 ON THE EXTINCT MAMMALIA OF 



seen in figure 8. The greater portion of the ramus forming the coronoid process is 

 lost, but from its remains it appears to have been peculiar. The border of the ramus 

 in the rodent jaw, and indeed in that of most animals, descending from the front of 

 the condyle to form the notch between it and the coronoid process, in the fossil under 

 examination proceeds downward and outward from the neck of the jaw externally, as 

 seen in figure 8. 



The mental foramen occupies the ordinary position below the fore part of the first 

 molar. 



The molar teeth in all the fossils under examination belonged to the permanent 

 series, and had advanced to an age at which they had all become inserted by the 

 fangs only. They are four in number, and have the same constitution as in the 

 Beaver, Agouti, Steneofiher, Ghalicomys, etc. 



In the specimens consisting of detached portions of jaws, in which the teeth are 

 best preserved, they present the following characters : 



In the upper molars the crowns successively decrease in size from the first to the 

 last, as in the Beaver. The crowns are cuboidal with rounded lateral borders, but 

 they have not so much of an outward curvature as in the Beaver. The worn tritu- 

 rating surfaces represented in figure 10, magnified two diameters, present a lateral 

 valley on each side as in the Beaver; the inner one directed outward, the outer one 

 inward and backward. The anterior division of the crown exhibits an elliptical islet 

 in the course of the inner valley, and usually an additional minute islet. The pos- 

 terior division of the crown exhibits from three to four minute islets included in the 

 bend of the external valley. 



In the lower molars, the crowns of the anterior three differ slightly in size ; the 

 first is the largest, the fourth the smallest of the series. Upon the worn triturating 

 surfaces, represented in figure 11, magnified two diameters, in the first tooth the inter- 

 nal valley divides the crown transversely, but in the other teeth it is converted into 

 an islet. The external valley extends obliquely backward. In advance of the 

 valleys the triturating surface usually presents an elliptical islet, but in the first 

 molar two small ones are substituted. Back of the valleys in the intermediate pair 

 of teeth there is a single transverse elliptical islet. In the first tooth there is an 

 additional small round one ; and in the last tooth there are one or two small round 

 ones. 



In the other specimens the teeth present slight differences, mainly dependent on 

 their more worn condition, one result of which has been the obliteration of most of 

 the smaller islets of the triturating surfaces. 



Measurements of the specimens of Pala>ocastor nehrascensis are as follow : 



Lines. 



Length of upper molar series, . . . . . .61 



Length uf lower molar series, . , . . . .71 



