GriS MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



often slender and delicate, but sometimes thick, squat, and clumsy. Hind 

 limbs never disproportionately developed. In one group, tlicrc is a broad 

 densely furred intermembral membrane. 



The Sciuridce share, in common with the other Sciuromorphs (Sciuromor- 

 ]>lta Alston rr Schiromorpha Brandt, emend.), the possession of perfect clavi- 

 cles; a nearly perfectly free fibula; small incisive foramina, not extending 

 into the maxillary ; the obsolescence of the outer wall of the pterygoid fossae, 

 and the absence of an intcrptcrygoid fissure; a small, naked muffle and a 

 cleft upper lip. The zygomatic arch is formed mainly by the malar, and the 

 angular portion of the mandible springs "from the lower edge of the bony 

 covering of the lower incisor" {Alston). 



The Sciuridce are distinguished from the other families of the Sciuro- 

 morphs by the following characters: — The Anomaluridce differ from the Sci- 

 uridce by the absence of postorbital processes, and in possessing a large ante- 

 orbital foramen, a narrower palate, which is contracted anteriorly and deeply 

 emarginate behind, and in having the molars non-tuberculate, flat-crowned, 

 and with loops of enamel. The Ischyromyidce differ from the Sciuridce in 

 having large anteorbital foramina, a sagittal crest, and no postorbital process. 

 The ILiplwlontidce have the grinding-teeth "rootless, simple, and prismatic", 

 and postorbital processes are absent, etc. The Castoridce (taking Castor as 

 the type) differ from the Sciuridce in lacking the postorbital processes; in 

 the molars being semi-rooted, with involutions of the enamel border; in the 

 form of the descending ramus of the lower jaw, the disproportionately large 

 hind limbs, fully webbed hind feet, flat, naked tail, etc. While in Castor the 

 anteorbital portion of the skull is Sciurine, all close resemblance to the Sci- 

 uridce here ceases. The Casloroididce differ from the Sciuridce through the 

 rootless, compound nature of the grinding-teeth, in the possession of several 

 Castorine features, and the structure of the pterygoid process, etc. 



The family Sciuridce embraces a considerable variety of forms, but they 

 so insensibly intergrade that it is almost impossible to separate them into 

 characterizablc subfamilies, the differences being wholly adaptative, and of no 

 great importance. The passage from the one extreme of Sciuropterus to the 

 other of Arctomys is by very gradual steps. The lithe, graceful, arboreal Sci- 

 uri differ but. little from Xerus and Tamias, forms still partially arboreal, 

 while Tamias and Spermophilus so intergrade that some species are doubt- 

 fully referable to the one rather than the other. The passage from Sper- 



