I ■ ; 



G38 



MONOGRAPHS OP NORTH AMERICAN RODENTIA. 



it 



i! 



' I 



of>cti slcntlcr and dclicntc, but sometimes thick, squnt, and clumsy. Hind 

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

 densely furred intertnembrul membrane. 



The SciuridcB i^hare, in common with the other Sciuromorphs (Sciuromor- 

 j)ha 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 fossie, 

 nnd 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" {Ahton). 



The Sciurida arc distinguished from the other families of the Sciuro- 

 morphs by the following characters : — The Anomalurida differ from the Set' 

 urula 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, 

 nnd with loops of enamel. The Ischyromyida differ from the Sciurida in 

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

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

 nnd postorbital ])rocesses are absent, etc. The Castorida (taking CaMor as 

 the type) differ from the Sciurida 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 Caxtor the 

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

 vrida here ceases. The Castoroidida differ from the Sciurida through the 

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

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



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

 so insensibly 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 Sciuropterus to the 

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

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

 while Tamicu and Spennophilua so intergrade that some species arc doubt- 

 fully referable to the one rather than the othor. The passage from Spfr- 



\n 



ml 



