638 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
often slender and delicate, but sometimes thick, squat, and clumsy. Hind 
limbs never disproportionately developed. In one group, there is a broad 
densely furred intermembral membrane. 
The Scturide share, in common with the other Sciuromorphs (Sciuromor- 
pha Alston = Sciuromorpha 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 fossze, 
and the absence of an interpterygoid 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” (Aéston). | 
The Sciuride are distinguished from the other families of the Sciuro- 
morphs by the following characters:—The Anomaluride differ from the Sci- 
uride 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 IJschyromyide differ from the Sciuride in 
having large anteorbital foramina, a sagittal crest, and no postorbital process. 
The Haplodontide have the grinding-teeth “rootless, simple, and prismatic”, 
and postorbital processes are absent, ete. The Castoride (taking Castor as 
the type) differ from the Sciwride 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, ete. While in Castor the 
anteorbital portion of the skull is Sciurine, all close resemblance to the Sc7- 
urid@ here ceases. The Castoroidide differ from the Sciuride through the 
rootless, compound nature of the grinding-teeth, in the possession of several 
Castorine features, and the structure of the pterygoid process, ete. 
The family Sciwride embraces a considerable variety of forms, but they 
so inseusibly intergrade that it is almost impossible to separate them into 
characterizable subfamilies, the differences being wholly adaptative, and of no 
great importance. The passage from the one extreme of Sciwropterus to the 
other of Arctomys is by very gradual steps. The lithe, graceful, arboreal Scz- 
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- 
