HODENTIA. 



375 



Esthonyx (fig. 212). Known only by the jaws and other fragments. The 



i. 2, c. 1, pm. 3, m. 3 



dental formula is: . _ -. The canines are well developed 



i. 3, c. 1, pm. 3, in. 3 



in both jaws. Of the upper incisors, the middle pair seems to be entirely 

 absent ; the second pair is much enlarged, growing from a persistent pulp 

 for a considerable period but not throughout life, and only destitute of 

 enamel in a narrow longitudinal band on the posterior or oral face ; the 

 third incisors are somewhat smaller than the enlarged pair. Of the 

 opposing teeth in the mandible it is also the second pair which exhibits 

 great enlargement, and this agrees with the large upper pair in all 



FIG. 212. 



Esthonyx burmeisteri ; portion of right upper and lower jaws, lateral aspect (a), 

 right upper dentition, oral aspect (b), and imperfect mandible, oral aspect 

 (c), two-thirds nat. size. L. Eocene (Wasatch Formation); Wyoming. 

 (After Cope.) 



respects ; the inner lower incisors are very small, and the outer pair 

 (no. 3) may be described as rudimentary. The species of this genus occur 

 in the Lower Eocene (Wasatch Formation) of New Mexico, and in the 

 Middle Eocene (Bridger Series) of Wyoming. They are all of small size, 

 the mandible of an average type (Esthonyx burmeisten) measuring about 

 O'l m. in length. One imperfect skull with molars from the London Clay 

 of Herne Bay, Kent, named Platychoerops richardsoni, is sometimes com- 

 pared with Esthonyx and believed to indicate the presence of this or an 

 allied genus in the Lower Eocene of Europe. 



