EODENTIA. 
171 
grooved, and is little or not at all inflected on the inner side of the shaft, 
while it is extensively so on the external face. 
No characters have yet been offered by which to distinguish the 
American species as representing a genus distinct from the Plesiarctomys 
gervaisii of the French Eocene. Bravard briefly distinguishes the genus as 
distinct from Arctomys in the greater thickness of the angles of the molars, 
which thus become tubercles. Only the mandible and mandibular teeth of 
the P. gervaisii are known. It has been found in the Upper Eocene of 
France, near Perreal, Apt. 
Plesiarctomys buccatus, Cope. 
Plato xliv", fig. 8. 
This Rodent is represented in the collections of the Survey by a right 
maxillary bone with adjacent parts of the cranium of one individual. 
Three molar teeth are perfectly preserved. The proportions are less than 
those of the P. delicatissimus, but larger than those of the P. undans. 
The malar process of the maxillary bone is largely extended in an 
antero-posterior direction as well as transversely, so that in the perfect skull 
it is probable that the orbit has a largely vertical direction. The malar 
process has a concave border, both anteriorly and posteriorly, being bounded 
by the orbit behind, and the large foramen infraorhitale anterius in front. 
Its inferior face is divided into tAvo planes by an angle, which extends out- 
Avard at right angles to the longitudinal axis of the skull. 
The outlines of the triturating faces of the molars are between quadrate 
and circular. They support four Ioav tubercles, tAvo each on the external 
and internal borders. The posterior inner is much smaller than the anterior 
inner, and incloses with the two external a concavity. On tile anterior and 
posterior borders of the croAvn there is a Ioav cingulum, but none on the 
inner and outer borders. Enamel smooth. 
Measurements. 
M. 
Lengths of the crowns of the three molars.-.- 0.0080 
Length of the crown of the third molar. . 0.0027 
Width of the same... 0.0030 
Length of the crown of the first molar... 0.0025 
Width of the same. ..... .. . 0.0030 
Antero posterior width of the base of the malar process of the maxillary bone 0.0050 
Horizontal diameter of foramen infraorhitale exterius . 0.0028 
