336 B. N. A. BOUNDARY COMMISSION. 

the carina on the enamel-face of the base of the crown at least; the apices being in 
each case broken away, Accordingly, the root exhibits no corresponding groove on 
its inner side, as is the case in C. arctatus. The form of the maxillary bone is also 
characteristic. In C. arctatus, this piece bears a longitudinal protuberance on its 
inner side, so as to have given the face great proportionate width. In C. stenopsis, 
this protuberance is much less pronounced; the inner face, instead of being nearly 
horizontal above, is curved abruptly downward, and a shallow horizontal face of no 
great width replaces the wide oblique cornice which extends from the alveolar border 
in C. arctatus, The remains indicate a species of the size of C. arctatus. 
From six miles west of First Rranch of Milk River, near latitude 49°. 
Measurements. 
M. 
Depth of the maxillary bone on a dental groove........ 2... cc cece sees ececceeece 0.0350 
Diameter of the en ary bone external to alveoli 2:8, bi ‘dein ars aco, he \hy Bhs 0)pase eck ae a ae EE 
Diameter of a tooth. a : Sw oie saeiah eb a'slie'e vcpsre,s slate biel ouitete ore eee 
COMPSEMYS, Leidy. 
This genus presents the characters of mys in its well-developed marginal bones, 
united to the costals by suture. The surfaces of the carapace possess a dense layer, 
which is sculptured in two of the known species. One of these, the C. victus, Leidy, 
has been found to have hada wide range in the West during the Fort Union epoch ; 
while a second has been found in corresponding strata near the northern boundary of 
Dakota, 
CoMPSEMYS oGMIUS, Cope. 
Represented in the collections of the British American Boundary Commission by 
portions of the carapace and plastron. These are massive, and indicate a species of 
large size. Asin other species of the genus, the external surface is a dense layer of 
cement or allied substance, which is sculptured with shallow pits. 
A portion of the costal bone is concave, and increases rapidly in thickness in 
one direction. The suture is coarse, but neither gomphosial nor squamosal. A 
portion of the plastron is thinner, not curved, and displays a very coarse median 
suture, in part squamosal in character. The sculpture consists of shallow pits, not 
wider than the low, smooth ridges which separate them. There are deep superficial 
grooves, marking the boundaries of dermal areas ; a feature in which this tortoise 
differs much from the Plastomenus coalescens, and resembles the species of Compsemys. 
Should marginal bones be found to exist in the C. ogmius, its reference to that genus 
will be further established. 
From six miles west of First Branch of Milk River, near latitude 49°. 
PLASTOMENUS, Cope. 
This genus has been discovered to cmbrace tortoises having characters of both 
Trionyx and Emys. The carapace is like that of the former, in the absence of articu- 
lated marginal bones, and the presence of a superficial cement layer, which is sculp- 
tured in various patterns. The plastron resembles that of some emydoid genera, 
but presents certain fontanelles indicating an incomplete grade of ossification. The 
species known to possess the typical structure are found in the Eocenes of Wyoming 
and New Mexico; and those here referred to it are all from the Fort Union or 
Transition beds of the Cretaceous, In none of them is the sternum so well-preserved 
as to exhibit the characters which should finally refer them to the genus Plastomenus. 
This is due to the fact that they, as well as other vertebrate remains from this 
horizon, are always much broken or dislocated, 
