158 BULLETIN OF THE 
We might infer from this that Symborodon can be clearly separated from 
Menodus by the absence of the lower incisors, accompanied by a narrowing of 
the symphysis ; but Professor Cope has recently described a new species, M. 
angustigenis, from the Swift Current Creek region,* which combines the nar- 
row type of symphysis with the presence of two incisors. The separation of 
these genera is rendered still more improbable by the parallelism which exists 
between the skulls from the Nebraska and Colorado localities, especially in 
respect to the conformation of the nasal bones and the horns. The genus 
Symborodon is however provisionally adopted at present to include the species 
with a narrow mandibular symphysis and no lower incisors. 
The genus Brontotheriwm, Marsh, cannot be distinguished from Menodus. It 
A iis a to 
, in the synopsis given by Pro- 
=s co) v 
fessor Marsh, as distinguished from Menodus with ? Pm. = - One of the 
rests in part upon the premolar formula, : 
lower jaws of the Princeton collection, however, has the premolar formula a= Ar 
demonstrating that the first lower premolar is a variable tooth, and cannot in 
this case be used in classification. The same rule applies to the second cone 
upon the last upper molar, the supposed generic character of Diconodon, 
Marsh. This is found in different species in all degrees of development, from 
a small prominence upon the basal cingulum to a well-developed cone (J. 
Proutit). 
Such characters as the invariable absence of lower incisors may subsequently 
be found to separate one genus of the Menodontide from another; but our 
present evidence goes to show that they simply characterize the extremes of a 
closely related series of animals, from the same horizon, of which the interme- 
diate forms are represented by numerous species. The safest basis of specific 
determination seems to be the correlation between the development and pro- 
portion of the horns and of the nasals, the rule being that where the horns 
are long the nasals are short, and conversely. The number of the teeth 
does not at present seem to be absolutely constant, even within the limits of 
the species. 
The following determination of the species in the Cambridge collection is, 
for the above and other obvious reasons, provisional. The classification can 
be finally settled only when the lower jaws and skulls are found in associa- 
tion. If, for example, a large number of forms of the M. coloradensis type of 
skull are found with but a single upper incisor, they will undoubtedly repre- 
sent a species distinct from both S. trigonoceras, Cope, and M. ingens, Marsh. 
M. coloradensis, Letpy, 1870. Syn. M. ingens, Marsh, Am. Journ. Sci. 
and Arts, 1874. S. trigonoceras, Cope, Synopsis New Vert. Col., 1873, p. 13. 
The type of this species, a snout with horns and nasals, was figured by Pro- 
* The Vertebrata of the Swift Current Creek Region of the Cypress Hills. Geol. 
and Nat. Hist. Surv. of Canada, 1886, p. 81c. 
+ Am. Journ. Sci. and Arts, od Ser., XI. 339. 
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