405 



appear to have been chiefly committed to the masseteric and pterygoid muscles. The zygoma- 

 tic arch, which extends from the squamosal to the premaxillary, is very broad below the orbit, 

 and is traversed externally by a ridge indicating the powerful origin of the masseter. The 

 antorbital vacuity and the maxillary depression, bounded externally by the two roots of the 

 zygoma, are larger than the orbits : the upper root of the zygoma is formed by a combina- 

 tion of the frontal, lacrymal, maxillary and malar bones. The slender extremities of the pre- 

 maxillaries terminate on nearly the same transverse line with the back part of the broad 

 nasals. These are bent down anteriorly, so as to form the sides of the external nostrils. The 

 deep sockets of the rootless teeth form protuberances at their bases, where the osseous case 

 has been absorbed, in some converting the socket into a canal open at both ends, the per- 

 sistent matrix of the tooth being attached to the periosteum, and protected by the contiguous 

 soft parts. In many respects the osteology of the genus Helamys resembles that of the Chin- 

 chilla tribe more than of the ordinary Jerboas. 



Hunter ian. 



Family Jerboida (Jerboas). 

 Genus Dipus. 



Dental formula : t \^, p {=}, m g= 16. 



2253. The skeleton of the Egyptian Jerboa (Dipus sagitta). 



The most prominent peculiarities in this skeleton are, the enormous acoustic bullse, the 

 small antorbital foramina beneath the large antorbital vacuities, the disproportionate size and 

 length of the hinder extremities, as compared with the fore, the large size of the ischium, as 

 compared with the ilium, and the coalescence of the metatarsals of the three middle toes into 

 one bone, as in Birds. The penis with its bone is attached to the pubis. The lower half of 

 the slender fibula is anchylosed to the tibia. 



Mm. Brookes. 



Family Ardomyid& (Marmots). 

 Genus Arctomys. 



Dental formula : t i, p 1=1, m ^=22. 



2-254. The skeleton of the Alpine Marmot (Arctomys Marmotta). 



The vertebral formula is : 7 cervical, 12 dorsal, 7 lumbar, 3 sacral, and 23 caudal. The 

 short and thick transverse processes of the atlas are bifid, indicating their constitution by 

 distinct pleurapophysial and diapophysial portions. The transverse processes of the last 

 cervical vertebra are perforated. The last five cervicals have no spines. These processes are 

 well developed on all the dorsals, that of the second not much exceeding the others hi length. 

 The accessory tubercle appears upon the diapophysis of the ninth dorsal, and divides in the 



