796 THE WHITE RIVEE FAUNA. 



M. 



Liength of second true molar - 0016 



Width of second true molar 0020 



Length of inferior molar series of a second iudivldnal 0128 



Depth of ramus at first premolar 0016 



Depth of ramus at first true molar 0035 



Depth of ramus at last true molar 0030 



These measurements indicate an animal of about the size of the com- 

 mon mole of this country {Scalops aquatious). The teeth are relatively 

 smaller, and the mandibular rami deeper and less robust. 



Peratherium tricuspis Cope. 



Merpetotieriavi trimtspis Cope. Synopsis of New Vertebrata from the Tertiary of Colorado (Miscell. 

 pub. U. S. Geol. Survey Terrs.), October, 1873, p. 5. Annual Report U. S. Geol. Surv. Terrs., 

 1873 (1874), p. 466; partim. 



PKiteLXII; figs. 10-11. 



This species as at present defined rests on a fragment of a lower jaw, 

 which supports the second and third true molars. These teeth are similar 

 to those of the Peratherium tricuspis, and possess, like some individuals 

 of that species, a prominent posterior inner cusp of the posterior triangle. 

 The character which distinguishes it from the typical species is the shal- 

 lowness of the mandibular ramus. The depth of this bone is not over half 

 that of some of the specimens of P. fuyax, with teeth of equal size, and 

 two-thirds the measurement of some others. This character is expressed 



in the following — 



Measurements. m. 



Length of second and third true molars 0040 



Width of third true molar 0010 



Depth of ramus at second true molar 002.5 



From the same locality as the H. fugax. ^ 



Peratherium huntii Cope. 



Plate LXII; figs. 12-16. 



Merpetotherium huntii Cope. Synopsis of New Vertebrata, Colorado, October, 1873, p. 5. Annual Kept. 

 U. S. Geol. Surv. Terrs., 1873 (1874), p. 466. H. sterensoiiii Cope, Synopsis Vert., Colorado, p. 6. 

 Ann. Report U. S. G. S., 1873 (1874), p. 467. Miothen gracile, Synopsis Vert., Col., 1873, p. 8. 

 Domnina gracilis Cope, Ann. Report U. S. G. S., 1873 (1874), p. 470. 



This is the smallest species of the genus known, and is readily distin- 

 guished from the P. fugax b}^ other characters than its minute size. These 

 are the uninterrupted series of premolars, which are without interspaces, 

 and the greater acuteness and elevation of the cusp on the inner side of 

 the posterior triangle. 



