576 



MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



.a 



of the acetabulum. The axis of the neural canal, like that of the spinous 

 processes, is gently convex ; but such is the disparity in size between the 

 two anterior and three succeeding centra that the face of the bone is strongly 

 concave. 



The caudal vertebne ore eleven in number, of which the first six are per- 

 forate with a neural canal, the remainder being imperforate. The series meas- 

 ures one and three-fourths inches, only about half of which length projects 

 from the body to constitute the tail-measurement of zoologists. There is no 

 trace of spinous processes on any of these coccygeals, the superior median 

 line being formed by the neural arches, apparently bifid in front, where the 

 forking pre-zygapophyses embrace the less-produced post-zygapophyses ; with 

 the subsidence of these formations on the 7th caudal, the vertebrae articulate 

 by their centra alone. The transverse processes, of undetermined homology, 

 are all distinct from each other as far as they occur at all ; they are largest on 

 the 2d caudal, whore they form thin horizontal plates as long as the body of 

 the bone, triangular in outline, the prominent angle posterior. The centra 

 successively decrease in size in every dimension, from first to last ; no pro- 

 cesses of any sort are fairly recognizable after the 9th vertebra,and the terminal 

 one is a minute, conical, acute bone about one-tenth of an inch long. 



Thorax. — ^This is strongly conical, much contracted anteriorly, dilated 

 and capacious posteriorly in that portion which lies behind the sternum, cov- 

 ering abdominal organs. The sternum is about two inches long and very nar-- 

 row, except at the top. It consists of six sternebers of nearly the same width 

 (excepting the manubrium), but successively graduated in length. The inter- 

 mediate pieces are similar to each other in their somewhat hourglass-shape, 

 being constricted in the continuity, expanded at each end ; the outer surface 

 has a more or less conspicuous longitudinal median ridge ; the inner surface 

 is flat. The last sterneber, or xiphoid, ends with a flat rounded extremity, 

 supplemented by a very slight cartilage, if any. The manubrium is much the 

 largest of the sternebers, with the best marked longitudinal ridge on its 

 exterior face, and much the strongest lateral processes for articulation with 

 the first pair of ribs, the bone being here more than two-thirds as broad as 

 long. There is a distinct episternal process, like a flat, oval disk, sessile on 

 top of the manubrium, bearing lateral facets for articulation with the clavicles, 

 which is eifected with the interposition of well-marked inter-articular fibro- 

 cartilages. 



