1883. ] The Extinct. Rodentia of North America. 57 
stead of being in three parallel lines, the middle line is only repre- 
sented by its extremities. At the middle of the crown the fos- 
settes of the internal line are incurved so as to be nearly in con- 
tact with the fossettes of the external line. 
HE Iiscomys Cope. 
Inferior molars four-rooted, the crowns supporting four cusps 
in transverse pairs. A broad ledge or cingulum projecting on the 
external side from base of the cusps. The inferior incisor com- 
pressed, not grooved, and with the enamel, without sculpture. 
Fic. 13.—a-d, lower jaws of Heliscomys vetus Cope; a, natural size p 4, c, d, €n- 
larged ; ¢, f, Eumys elegans Leidy, natural size ; e, cranium from above; J, left ramus 
of lower jaw, external side. All from the White River epoch of Colorado; ‘Original. 
This genus is only represented by a small number of speci- 
mens, which are mandibular rami exclusively. Its special affini- 
ties therefore cannot be ascertained, and even its general position 
remains somewhat doubtful. There is some probability, however, 
that it belongs to the Myomorpha, as the type of dentition is much 
more like that of the genera of that group than those of the Sci- 
uromorpha. To the Hystricomorpha it does not belong. 
As compared with known genera of Myomorpha, it is at once 
separated from many of them by the presence of a premolar tooth. 
Among recent genera of this sub-order, Sminthus possesses this 
tooth in both jaws, and Meriones in the upper jaw only. It is 
present in both jaws in the Sciuromorpha generally. The tuber- 
cles of the teeth resemble those of the Muridz, but their disposi- 
tion is unlike that of any existing North American genus. re- 
mote approximation to it is seen in the genus Syllophodus of the 
Bridger Eocene formation, where there are four subquadrate 
molars with tubercles; but the latter form two transverse crests, 
with an additional small intermediate tubercle, and the wide cin- 
gulum is absent. 
But one species of Heliscomys is known, the H. vetus, from the 
White River epoch of Colorado. It is not larger than the domestic 
mouse (Mus musculus). 
(To be continued.) 
