152 
MESODONTA. 
Opisthotomus astutus, Cope. 
Loc. cit., p. 16. 
Plate xlv, lig. 9. 
Established on two posterior lower molars, which were found in inime- 
diate association, but not attached to the jaw. The penultimate molar has 
a convex cingulum in front connecting the two cusps, and a median poste¬ 
rior cusp of small size. The last molar has a wide ledge in front, and the 
anterior cusp is on its outer side. It is separated by a valley from the 
middle cusp, which is the most elevated. Directly on its inner anterior 
aspect, a stout cusp is attached, and connects by a low ridge with the ante¬ 
rior. The posterior cusp is elevated, conic, and median, and is connected 
with the central cusp except at its apex. There is no heel behind it, nor 
any cingulum on this or the anterior molar tooth. The enamel of both 
teeth is nearly smooth. 
Measurements. 
M. 
Length of the crown of the median lower molar... 0.0065 
Width of the same....... 0.0060 
Length of the crown of the posterior lower molar.. 0. 0085 
Width of the same in front. ..... 0. 0050 
Length to the middle cusp.. 0. 0045 
Elevation of the middle cusp.. . 0.0050 
Opisthotomus flagrans, Cope. 
Plate xlv, fig. 8. 
Loc. cit., p. 16. 
A larger species belonging to this genus, is represented by a portion 
of the lower jaw, from which the molars are broken, excepting the last. 
Idle outline of the base of the crown of the latter is elongate-subtriangular, 
the base of the triangle being anterior. The anterior fom’th of the crown 
is a ledge with angulate border, and a tubercle at the outer anterior corner; 
the remainder of the crown consists of three elevated cusps, an external 
alternating with an internal, and a median posterior. The inner and outer 
cusps are rather obtuse, and are separated by a deep notch; the posterior 
cusp is much lower, is obtuse, and has a still smaller tubercle at its inner 
basis. The surface of the heel is oblique, for the external anterior tubercle 
sends a ridge to the internal median cusp, so that the line of elevation of the 
crown is a zigzag of three limbs. 
