188 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. IVol.Vl. 



is entirely surrounded by a ciugulum, and its median line is elevated 

 into a blade, which is continuous with the posterior edge of the princi- 

 pal cusp. Both edges of the anterior tubercle are also trenchant. The 

 two cusps of the anterior inner tubercle of the first and fourth molars 

 are well developed, but on the second molar there is but one cusp. This 

 is probably a character to be relied on in distinguishing the species from 

 the P. chacensis. No external basal cingula ; enamel smooth. 



Measurements. 



M. 



Lengtli of last four molars 0218 



Length of fourth premolar 0068 



Elevation of fourth premolar 0040 



Length of last true molar 0057 



Depth of ramus at first true molar 0070 



As is the case with the species of Pantolestes already known, the P. 

 pecans seems to have been rare. 



19. MiCROSYOPS SPEIEIANLTS Cope. 



American Naturalist, 1879, p. 908. 



Established on a portion of a mandibular ramus which contains the 

 three true molars in perfect preservation. As the number of premolar 

 teeth is unknown, its reference to this genus is provisional only. The 

 last true molar has the form of that of the M. gracilis Leidy. It is dis- 

 tinguished by its very small size, siuce it is considerably less than the 

 M. vicariiis {H.f minusculus), and by the equality in size of the molars. 

 The heel of the third molar is very small, and the two cones of the inner 

 side of the crowns of all the molars are acute. The external crescents 

 are very well defined, the anterior sending a horn round the anterior 

 extremity of the crown. The posterior is connected with the corre- 

 sponding internal tubercle by a median conic posterior tubercle. Length 

 of true molar series, .008 ; length of second molar, .0026 ; width of sec- 

 ond molar, .0022 ; length of last true molar, .0025 ; width of last true 

 molar, .0016 ; depth of ramus at second molar, .0043. Dedicated to my 

 friend Mr. Francis Speir, of Princeton, ]S". J., who, in connection with 

 Messrs. Scott and Osborne, has made important additions to our knowl- 

 edge of the Eocene Vertebrata. 



20. MiCROSYOPS GRACILIS Leidy. 

 Eepresented by numerous jaws. 



21. MiCROSYOPS SCOTTIANUS Sp. nov. 



A nearly entire left mandibular ramus is all that I have seen of this 

 species. The crowns of the fourth and sixth molars furnish the only 

 dental characters available, but the number and forms of the bases of 

 the others are readily ascertainable. 



The ramus of the jaw is more slender than in ill. gracilis, and the last 

 true molar has quite a different form. Instead of being shorter than in 



