DIPODIDsE 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. Entoptyckus, from the 

 Miocene of the United States, is an allied genus, with broad incisors 

 and rootless molars. 



Subfamily Heteromyinae. — 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. 1 — This genus is characterised by the rootless molars. 

 It is best known by D. plullipsi, 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 2 and Heteromys.' 3 — 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 ; Avhile 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 Dipodid.e. 



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 Sminthinse. — Molars rooted ; p £, m f . Skull with 

 the infraorbital vacuity widest below, and the incisive palatal 

 foramina long. Limbs short. Palaearctic. 



tfminfhus* — 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 

 Muridce, but was transferred in 1887 to the present family by 

 Dr. H. Winge. 



1 Gray, Ann. Mag. Nat. Hist. vol. vii. p. 521 (1840). 



- Wied, Nova Acta Ac. Cues. Lcop.-Car. vol. xix. pt. i. p. 369 (1839). 



3 Desmarest, Mammalogie, p. 313 (1820). 



4 Keyserling und Blasius, Wirhcltliicrc Europ. p. 38 (1840). 



