SACCOMYID^}— DIPODOMYINZE— DirODOMYS. 521 



Perognathus by the structure of the ear. The present species may be called 

 O. longimembris, as the length of the bind limbs and tail is its specific char- 

 acter in comparison with O.jlava. 



Subfamily DirODOMYINJ]]. 



= Dipodomys (genus), of Authors. 

 = Dipodomyincr, Coues, Proc. Pbila. Acad. 1875, 277. 



=Macrocolini, Brandt, libit. Kennt. Siiug. Russl., vierte Abh., Fortsch. Grnpp. Nagcr, 1855, p. 231 and 

 p. 311. 



The characters upon which this group is based having been given on a 

 preceding page (p. 494), we may at once proceed to consider the single 

 genus by which it is represented. 



Genus DIPODOMYS, Gray. 



= Dipodomys, t Gray, Ann. Mag. Nat. Hist, vii, 1841, 521. (Type D. phillipii.) 

 = Macrocolus, * Wagner. Archiv fur Naturg. 1846, Bd. i, 172. (Type M. halticus.) 



Chars. — (a. Cranial.) — Skull light, thin, papery, depressed, broad behind 

 with swollen curves, tapering in front; rostrum acuminate, produced beyond 

 incisors; no interorbital constriction; palate plane ; occipital surface deeply 

 emarginate. Zygomata straight, thread-like, depressed to palatal plane; abut- 

 ting against tympanies. Anteorbital foramina represented by a circular per- 

 foration of the front of the maxillarics. Large excavated lachrymals. Parietals 

 triangular; an elongate interparietal embraced between forks of occipital. 

 Squamosals reduced to small plates bounding the orbits posteriorly ; other 

 elements of the temporal bone extraordinarily developed, thin, and bladdery, 

 their sinuses of nearly as great capacity as the cerebral cavity ; the mastoids 

 especially enormous, constituting nearly all the occipital and the greater part 

 of the superior (behind the parietals) surfaces of the skull ; the tympanies 

 proportionally inflated, with large non-tubular orifice of meatus ; the petrosals 

 bullous, their apices in contact across the median line below the basisphenoid. 

 Tympanies, mastoids, and parietals entering orbits. Occipital singularly 

 reduced and narrowed, bent into three planes at right angles; supra-occipital 

 bifurcate to inclose an interparietal; paroccipitals narrow, flange-like ; basi- 



* Three special papers, substantially the same, differing in some particulars, viz.: — 

 " Oq a new Genus of Mexican Glirine Mammalia." < Ann. Mag. N. H. vii, 1841, pp. 521, 522. (The 



original notice; genus named on p. 521 ; species D. phillipii, same page.) 

 "On a New Glirine Animal from Mexico." < Rep. Brit. Assoc. Adv. Sci. for 1841, 1842, Trans, of the Sec- 

 tions, p. 70. (Same orthography of name.) 

 "On anew Glirine Animal from Mexico." < Amer. Journ. Sci. and Arts, xlii, 1842, pp. 334, 335. (Editorial 

 reproduction of the last ; name D. phillipsii on p. 335.) 

 t Special paper : — 

 "Macrocolus, eine neue Nagergattung ans der Familie dcr Springer." < Arch, fur Naturg., zwolfter 

 Jahrg. Bd. i, 1846, pp. 172-178. (Species M. halticus named, p. 176.) 



