316 EODENTIA, 



contraction of the bodies into a well-marked ridge occurs in tliese rodents in tlie 

 sacral region. The sacrum, unlike that of the Iluridce, is very compact and 

 strong, the pleiuapophyses being considerably expanded and broadly applied to the 

 ilium. 



In j5. badi'us, Hodgson, only two vertebrse are applied to the ilium, the tliird 

 vertebra, although assuming the form of a sacral, is quite free and followed by 

 another similar segment ; so that if these two are regarded as pseudo-sacral, there 

 are only sixteen caudal vertebrae. In R. pruinosus, the third sacral vertebra 

 partially touches the ilium, and is amalgamated posteriorly with a pseudo-sacral 

 vertebral element resembling itself; so that, leaving these out of view, there 

 are 19 caudals. The pseudo-sacral element exists to give support to the thickened 

 base of the tail. In these respects the skeleton of Bhizoniys resembles that of 

 Siplmeus. 



Broad transverse processes are well developed on the first five caudal vertebrae, but 



they disaj)pear on the seventh, or are represented by a lateral ridge, as far back as the 



seventeenth. In M. hadms, Hodgson, the transverse processes are distinctly visible as 



far back as the ninth vertebra, and tlieii' rudiments can be traced even to the thirteenth. 



In both these species, these processes are horizontally expanded. Also in both, the 



neural canal is perfect on the first four true caudals, and hsemapophyses are developed 



from the sixth to the twelfth vertebra. Metapophyses occur from the first to tlie 



fifth caudal, and are well developed. The bodies diminish slightly in length from the 



first to the fifth caudal, beyond which they lengthen to the ninth, after which they 



again decrease in length. Eight ribs are dnectly attached to the sternum, which 



consists of seven to eight osseous pieces, the last long and narrow, and occasionally 



amalgamating with the smallest of the segments which immediately precedes it. It is 



capped by a broad halbert-shaped xyphoid cartilage resembbng the manubrium 



in form. The manubrium at its lower end, and the various segments of the meso- 



sternum, have each a well-marked epiphysis, and the sternal tips of the rib cartilages 



are caj)ped with Httle ossicles in M. bacUus. The clavicle is strong and slightly 



outwardly and downwardly curved in its inuer half, this head of the bone being large 



and rounded, while its acromial end expands, and is flattened from above downwards. 



In one skeleton of R. pruinosus a small ossicle occurs at the sternal end of the 



clavicle. In R. bacllus there are only six sternal segments and seven sternal ribs. 



The manubrium in both species resembles that of Siplmeus, and is short and much 



expanded, so much so that it is broader than long, and is rounded anteriorly ; hence 



it is very different from the form of this bone in Bathyergus and Georychiis. It 



has a ventral ridge, rather well marked in one female, but nearly obsolete in a male 



skeleton. In a female, also, the manubrium is longer than broad, the lower end or 



shaft being well defined, while in a male it is extremely short — nay, almost 



absent. Although these two bones are markedly distinct, there can be no doubt 



of the specific identity of the two sexes, as they were both killed together. 



The manubrium in a female of R. hadius has the same form as in the male 



of both species. 



