882 THE WHITE KIVER FAUNA. 



investment, but on wearing becomes connected with it. They remain dis- 

 tinct to an advanced age of the animal, since the grooves which bound 

 them descend far into the alveoU before disappearing. Anotlier character- 

 istic of the specimen at least, is seen in the intervention of a wide isthmus 

 between the two principal lobes of each molar, or, in other words, of a 

 naiTowed portion of the second column between its transverse portion and 

 the anterior column. The result is that the triturating surfaces of the pos- 

 terior column of the molar has a quadrilobate outline; one lobe anterior, 

 one posterior, and two lateral. 



The tuberosity of the inner side of the ramus, which incloses the 

 incisive alveolus, extends to below the second molar. Its surface, and a 

 portion of that above it, is roughened with small punctifoi-m impressions. 

 The external face of the ramus is smooth and somewhat convex anteropos- 

 teriorly and vertically. The anterior border of the masseteric fossa is not 

 prominent, as in P. turgidus, is regularly convex, and extends to the line of 

 the posterior border of the penultimate molar. 



This species i*ests on characters which I have observed to be transi- 

 tional in the P. haydeni, and I have attended to the possibility of the indi- 

 vidual which has furnished them being a similarly immature P. turgidus. In 

 a considerable number of specimens of the latter no approach to the present 

 one is exhibited ; the latter is a fully-grown animal, and its characters would 

 remain after long attrition of the teeth. 



Measurements. 



M. 



Length of molar series 016 



Length of median three molars 010 



Width of median molar 003 



Depth of ramus at median molar Oil 



This species is rather larger than the prairie-marmot {Cynomys ludovi- 

 cianus). 



Pal^olagus turgidus Cope. 



Paleontological Bulletin No. 16, p. 4. Annual Report U. S. Geol. Surv. Terrs., 1873 (1874), p. 479. 

 Tricium jyanierise Cope, Pal. Bulletin No. 16, p. 5. 



Plates LXVI, fig. 28; LXVII, figs. 13-27. 



This is the largest species of the genus, and after the P. haydeni the 

 most abundantly represented in collections. At the locality which furnished 



