382 ANATOMY OF VERTEBEATES. 



rudiment of a third trochanter. The fibula is anchylosed to the 

 tibia. A remarkable accessory ossicle, articulated to the tarsal os 

 naviculare, projects inward like an accessory or sixth digit of the 

 hind foot. As in other burrowing animals, the lumbar and pelvic 

 regions are narrow. 



In the Marmot (^Arctomys) the clavicles are complete and 

 strong. The acromion is long and bifurcate, the anterior division 

 curves to the clavicle. The humerus shows a thick, but not pro- 

 minent, deltoid ridge : it is perforate between the condyles^ and 

 above the inner condyle. The antibrachial bones admit of 

 rotation. In the femur there is a rudiment of the third tro- 

 chanter : the tibia is not confluent with the fibula. In all the 

 Kat-tribe the clavicles are entire : the distal part of the fibula 

 coalesces with the tibia. In the Black Rat (^Mus rattus) the 

 deltoid ridge is angular, and commences near the upper end of 

 the humerus, which is imperforate at the lower extremity. A 

 strong ridge represents the third trochanter of the femur. There 

 is a fabella behind each condyle. In an Australian liat {Hajja- 

 lotis albipes) the humerus is perforated between the condyles. 

 The radius and ulna are moveably united. There is a third 

 trochanter in the femur. In the Hydromys the deltoid ridge 

 projects from the fore part of the proximal half of the humerus, 

 and is prominent below. The humerus is imperforate. The ulna 

 sends a process to abut against the radius across the middle of 

 the interosseous space. The fore foot is pentadactyle, but the 

 pollex does not exceed the length of the metacarpus of the 

 index. The femur has a third trochanter and a fabella behind 

 each condyle. The hallux extends to the second phalanx of the 

 next toe. The strength of the hinder half of the skeleton, with 

 the size of the hind extremities, contrasts with the slenderness of 

 the fore part in most Rats, and especially in this large Australian 

 aquatic kind. 



Among the leaping Rodents the following noteworthy cha- 

 racters of the limb-bones are seen in the Great Jerboa of the 

 Cape {Helamys). The lower costa of the scapula forms an acute 

 angle with the base, and the infraspinal fossa is much broader 

 than the supraspinal one, the spine of the scapida curving toward 

 the upper angle. The acromion is moderately long and slender, the 

 tuberosity answering to the lower division of that in the Caviadce. 

 The clavicles are strong, and curved backward at their outer half. 

 The humerus is perforated at the inner condyle, but not betAveen 

 the condyles. The bones of the fore-arm have a long and wide 

 interosseous space, and allow of free pronation and supination. The 



