2)7'^ SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



succeeding segment is wider than the one in front. The third and 

 fourth mesosternal segments are completely ankylosed so that the 

 whole mesosternum is composed of but three separate pieces instead 

 of the usual four. The xiphisternum is long and slender and about 

 equally expanded at each end. In all the preceding sections the sixth 

 and seventh pairs of ribs are attached to the last piece of the meso- 

 sternum in the angle between it and the xiphisternum. In case of 

 the form of sternum just described the sixth rib is the last attached 

 directly to the sternum, the seventh being attached to the cartilage 

 of the sixth near the point where the latter joins the mesosternum. 

 This type of sternum is found in the skeleton of Liiiinolagus pahidi- 

 cola. (PI. xcvi, 5.) 



7. In this section the sternum is very characteristic and resembles 

 almost exactly that of Ochotona. The anterior portion of the pre- 

 sternum is very much expanded and flattened dorso-ventrally. To 

 the outer posterior angles of this expanded portion the first pair 

 of ribs is attached. The rest of the presternum is long and narrow, 

 as it is throughout the Leporidse, but devoid of any ventral keel. A 

 slight ridge indicating a keel is seen on the ventral face of the 

 expanded portion. The mesosternum in general is very much like 

 the same structure in Liuinolagiis just described. The first and 

 second segments, however, are subec|ual and relatively narrower, the 

 third and fourth segments are subequal in length, but the fourth is 

 broader, both are relatively wider than they are in Liinnolagus and 

 as in that genus are firmly ankylosed. The xiphisternum is long 

 and rather stout ; it is about equally enlarged at either end. It is but 

 a trifle shorter than the whole mesosternum and decidedly longer 

 than the presternum. As in Limnolagus the seventh pair of ribs 

 does not articulate with the sternum, but with the cartilages of the 

 sixth pair. The above is the sternum of Roincrolagus. (PI. 



XCVI, I.) 



8. In this group the presternum is considerably enlarged in its 

 anterior third, to about the same extent that it is in Romerolagits. 

 Its different shape is best seen by consulting figures I -and 4, 

 plate XCVI. The mesosternum consists of four distinct segments, 

 the first two subequal in length, compressed laterally ; the third seg- 

 ment is slightly shorter, not laterally compressed ; the fourth segment 

 is very short and cartilaginous. The xiphisternum is very short, 

 much shorter than the presternum ; its anterior end is considerably 

 enlarged. Apparently six pairs of ribs only articulate directly with 

 the sternum. This type of sternum is found in A^csolagiis (Major, 



99, pi. 39, fig. 18). 



