326 \ A NEW MYLODON. 
the former much more nearly in a transverse plane than the latter. The two 
facets meet at a point slightly posterior to the vertical axis of the tooth. The 
long diameter of the tooth, which coincides with the axis of the toothrow, is 
25 mm., the breadth 16 mm. 
The second lower tooth has a broad squarish external lobe, and two narrower 
internal lobes, of which the posterior is the longer and beveled by contact with 
upper molar 3. Compared with M. harlani the anterior lobe seems better defined 
by a fairly deep sulcus, which in the latter species is merely a shallow concavity, 
as figured by Leidy (1855). It corresponds fairly well with the tooth figured 
by Cope (1895) as the second inferior molar of M. renidens, but is smaller. The 
discrepancies in outlines, however, seem trivial. 
The third lower tooth has roughly the outline of a boot, of which the toe 
and heel form respectively the anterior and the posterior external lobes, and the 
boot leg the longer internal lobe with nearly parallel sides. This lobe is much 
better marked off than in M. harlani or the two species described by Cope as 
M. renidens and M. sulcidens, in which the outline is roughly a parallelogram 
with rounded angles. Indeed, it is difficult to see how these last two can be 
distinguished from M. harlani if small discrepancies are discounted. M. sulcidens 
was named on the basis of a separate third lower molar, closely resembling that 
of M. renidens but with the dimensions of the corresponding tooth of WM. harlani. 
According to its describer ‘‘the internal extremity of the crown is beveled on 
the posterior border, so that an obtuse ridge characterizes the posterior side of 
the crown, which is separated from the posterior border of the external face.” 
The same condition is found in our specimen and is the result of wear against 
the upper third molar. The differences claimed as separating M. sulcidens 
from WM. harlani, must, I think, be considered merely individual. The greatest 
length of the third lower molar is at an angle of nearly 45 degrees to the tooth 
row, and in our specimen measures 35 mm., practically as in M. harlani (33 mm.) ; 
the breadth across the two external lobes is 23 mm. as against 20 in the latter. 
The fourth lower molar is very different from that of any species hitherto 
known, in its extreme elongation and lateral compression. As in M. harlani 
and Paramylodon it consists of an anterior and a posterior lobe connected by 
an isthmus. Each lobe has an internal heel. The long diameter of these lobes 
is nearly transverse to the axis of the toothrow in M. harlani and in Paramylodon, 
but in the specimen here described is at only a slight angle to this axis. On the 
external side there is a slight concavity opposite the first internal lobe, but this 
is much more marked in M. harlani and in Paramylodon is a sharp depression. 
