MESODONTA. 217 



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 corresponding internal tubercle by a median 

 conic posterior tubercle. Length of true molar series, .008 ; length of 

 second molar, .0026 ; width of second 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 Spier, of Princeton, N. 

 J., who, in connection with Messrs. Scott and Osborne, has made impor- 

 tant additions to our knowledge of the Eocene Vertebrata. 



Valley of the Big Horn River, Wyoming. J. L. Wortman. 



MiCROSYOPS ELEGANS Marsh. 



Limuotherium elegans Mr.rsb, Amer. Jouru. Sci. aud Arts, 1871, ii, p. 43, fide Leidy. 

 Miero8)jops gracilifi Leidy, Proceed. Acad. Phila. l6~-2, p. HO. 



This, the largest species of the genus, left abundant remains in the beds 

 of the Wind River, Wyoming. The specimens in my possession agree 

 closely with the descriptions and figures given by Leidy. The cusps of 

 the molars are well developed, and the angles bounding the heels are dis- 

 tinct. The fifth lobe or heel of the last true molar is quite small, and is on 

 the inner side of the median line. 



Measurements. 



M. 



Length of five last molars on liases 0206 



Length of last two premolars on bases 0070 



_. ^ „ , <• ^ i, 1 (anteroposterior 0030 



Diameter of base of fourth premolar < ^ „,„„ 



( transverse 0030 



anteroposterior 0040 



Diameters of second true molar < ^ „,,.,- 



I transverse 00J5 



T,. ^ i. , i ^ , C anteroposterior 0050 



Diameters of last true molar < ^ „„„. 



( transverse 003o 



Depth of ramus at third premolar 0090 



Depth of ramus at third true molar 0100 



MiCROSYOPS SCOTTIANUS Cope. 



Bulletin U. S. Geological Survey Terrs., 1881, p. 188. 



Plate xxiv a, fig. 21!. 



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. 



