'200 DR. J. E. GRAY ON SACCOMYIN^. [Mar. 12, 



The SaccomyincB are divisible into two tribes by the form of the 

 grinders. The coloration of the species of each tribe is so much 

 alike that it is almost impossible to distinguish them except by other 

 characters. 



The species of Heteromyina are only to be separated by the form 

 and surface of the upper cutting-teeth and the form of the hairs of 

 the fur, and the comparative length and hairiness of the tail. 



It would be well if the hairs of the fur of each of the species of 

 Heteromys a.nd Peroffnathus were figured under the microscope; but I 

 will leave that to be done by some younger zoologist. 



There is considerable difference in the form and size of the ridges 

 of the crowns of the grinders in the different species, which require 

 to be figured and described at length. 



1 . Grinders rootless; upper cutting-teeth with a longitudinal groove. 

 Fur soft, long, without any bristles. Dipodomyina. 



1. DiPODOMYS, Gray. 



Dipodomys, Gray, Ann. & Mag. N. H. 1846, p. 521; Aud. & 

 Bachm. N. A. Quad. iii. 137, 1853 ; S. Baird, Mamm. N. A. 406. 



? Macrocolus, Wagner, Wiegm. Arch. i. 176, 1846 ; Abhand. K. B. 

 Akad. Miinch. 1848, x. t. 7. 



Wagner says his animal has no external cheek- pouches ; but, as 

 Dr. Baird observes, " the coincidence in every other respect is so 

 intimate as to render it almost certain that the cheek-pouches must 

 have been overlooked, especially as we are particularly informed that 

 the specimen was in a very defective condition as preserved in alco- 

 hol." — Spencer Baird, Mamm. N. A. 409. 



1. Dipodomys phillipsii, Gray, Ann. & Mag. N. H. vii. 521, 

 1840 ; S. Baird, Mamm. N. A. 412. 



B. phillipsii, Aud. & Bach. N. A. Quad. iii. 137, t. 130, 1853. 

 Macrocolus halticus, Wagner, Wiegm. Arch. i. 172, 1846; Ab- 

 hand. K. B. Akad. 1845, p. 319, t. 7 (skeleton). 



JIab. Mexico, Real del Monte {John Phillips). B.M. 



Var. Larger, darker, the colour more distinctly marked. 



D. agilis, Gambel ; Spencer Baird, Mamm. N. A. 414. 



Hah. California {Bridges). 



" Found in dense thickets, Maccartysville, Santa Clara county. It 

 lives in the nests made by the Neotoma. Caught in traps during the 

 night, baited with wheat" {Bridges). The tail of the variety is shorter, 

 compared with the size of the body, than that of the stuffed specimen ; 

 but there is no knowing how much the skin of the body may have 

 contracted before it was preserved. 



Dr. Spencer Baird describes two other species of the genus, chiefly 

 depending on the absolute length of the hind foot and the length of 

 the tail as compared with the lengtli of the body. He believes that 

 there are at least two good species — one with a long tail and longer 



