36 Mammalia. Sectorialia. 



is the case, the radius and ulna are generally found in close contact, 

 or actually anchylosed ; the humerus is generally perforated, but 

 only letiveen the condyles ; and the tibia and fibula are distinct. On 

 the other hand, when the clavicles are complete {eg. in Castoridae, 

 Sciurida3, Muridae, &c.) the bones of the fore arm are generally dis- 

 tinct (ex. Pteromys); the humerus has the inner condyle perforated, 

 and, the tibia and fibula are usually coalesced. In some Rodents 

 the spine of the Scapula is extended into an acromion at some distance 

 beyond the glenoid cavity and there developes a retroverted process : 

 the intermuscular cristse on the subscapular surface are usually well 

 marked. In Pteromys a strong accessory cartilage projects from the 

 ulnar side of the carpus, which aids in supporting the lateral fold of 

 integument there developed. 



As in other burrowing animals, the lumbar and pelvic regions 

 of those llodents that burrow are narrow. In the Guinea-Pig 

 the ossa innominata can glide on the sacro-iliac joints, the sym- 

 physial ligament being extensible. In most Rodents i\ie femur has 

 a slender neck and lofty trochanter. 



yi. SECTORIALIA. 



All the bones of the Skeleton are remarkable for their whiteness 

 and compact structure. 



The SJcull. The upper contour of the Skull is generally rather 

 straight : an Occipital crest is always developed ; but though the 

 Temporal ridges usually meet, they by no means always form a 

 Sagittal crest of any size. Most Camivora have the Orbit incomplete 

 behind, and the Orbital and Temporal fossa3 continuous. The 

 Maxillaries usually contribute to form the anterior border of the 

 orbit. There is generally an Antorbital foramen ; its size is indicative 

 of the size of the nerve (2nd div. of V.) supplying the well developed 

 whiskers. The Frontal seldom, if ever, sends any process to join the 

 Malar. The Malar generally bifurcates posteriorly, and it alone in 

 most cases forms the post-orbital process ; the squamosal element 

 (zygoma) not extending so far. The Zggomatic-process is generally thick 

 and large, and forms an expanded arch. The Mandible is short and 

 strong ; the articulating condyle is much elongated transversely, and 

 locked in a deep cavity of corresponding form in the squamosal, so as 

 to admit of no lateral motion, and almost restricting the movements 

 of the mandible to one plane, for biting and dividing, not pounding 

 the food : the Coronoid process is broad and high, to give insertion 



