DIPODIDE 479 
with numerous varieties found all over Canada and North America 
west of the Rocky Mountains. Remains referred to an existing 
species occur in the Pliocene of Oregon.  Lntoptychus, from the 
Miocene of the United States, is an allied genus, with broad incisors 
and rootless molars. 
Subfamily Heteromyine.—Incisors narrow; mastoid appearing 
largely on the top of the skull; eyes and ears moderate or large ; 
hind limbs and tail elongated. Habits terrestrial. 
Dipodomys.1This genus is characterised by the rootless molars. 
It is best known by D. phillipsi, the Kangaroo-Rat of the desert 
regions east of the Rocky Mountains, having habits like those of 
the Jerboas. The typical forms have four toes in the pes; but in 
others, which it has been proposed to separate as Dipodops, there 
are five: D. ordi and D. agilis belong to the latter group. 
Perognathus? and Heteromys.2—In both these genera, which are 
represented by species of very small size, the molars are rooted ; 
the latter being distinguished by the presence of flattened spines 
mingled with the fur, and having species ranging into South 
America. According to Dr. C. H. Merriam the forms described as 
Cricetodipus are not separable from Perognathus ; while Dr. Coues 
considers that Saccomys was founded upon a species of Heteromys. 
Pleurolichus, from the Miocene of the United States, is regarded as 
an extinct genus allied to Heteromys. 
Family DIpopID&. 
Terrestrial forms usually with four upper cheek-teeth, and typi- 
cally with the following characters. Incisors compressed; molars 
with transverse enamel-folds ; infraorbital vacuity of skull (Fig. 7, 
p. 37) large and rounded ; jugal ascending in front to the lachry- 
mal; and the mastoid part of the auditory bulla usually very large. 
Subfamily Sminthinee.—Molars rooted ; » 3, m 3. Skull with 
the infraorbital vacuity widest below, and the incisive palatal 
foramina long. Limbs short. Paleearctic. 
Sminthus.s— Represented by the Rat-like S. vagans from Northern 
Europe and Asia, in which the ears are rather long and pointed, the 
tail is covered with short hairs and nearly as long as the body, 
while the molars present a somewhat complicated pattern. This 
genus has generally been regarded as an aberrant member of the 
Muride, but was transferred in 1887 to the present family by 
Dr. H. Winge. 
1 Gray, dan. Mag. Nat. Hist. vol. vii. p. 521 (1840). 
2 Wied, Nova Acta Ac. Ces, Leop.-Car, vol. xix. pt. i. p. 869 (1839). 
3 Desmarest, Mammalogie, p. 318 (1820). 
+ Keyserling und Blasius, Wirbelthiere Ewrop. p. 38 (1840). 
