SKELETON OF RODENTIA. 381 



The Beaver, fig. 230, is a member of that great division of the 

 Kodentia in which the clavicles are complete : the acromion scapnlas 

 bends toward and joins that bone. In the humerus the deltoid ridge 

 has a tuberosity : both the intercondyloid space and the internal 

 condyle are imperforate : a coarse cancellous structure occupies 

 the middle of the shaft. The radius and ulna are distinct. The 

 femur shows the slender neck and lofty trochanter common to 

 most Kodents ; it has a third trochanter, and has no medullary 

 cavity. The rotular surface is distinct from that of the condyles. 

 A section of the tibia and fibula also shows the absence of that 

 ca\ity, and the complete confluence of the compact walls of the 

 two bones at the lowar third of the fibula. The projecting part 

 of the calcaneum is depressed. The toes are longer and stronger 

 than the fingers, they support a broad foot which is webbed, and 

 the second toe has a double oblique nail or broad claw. 



In our Water- Yole {Arvicola cmijihihia) the acromion of the 

 scapula is long and bent downward ; its inferior process is feebly 

 developed. The deltoid process of the humerus is prominent 

 and well-defined, compressed, and bent downward. There is 

 a minute perforation between the condyles, but none above the 

 inner one. The bones of the fore-arm are in contact and 

 closely united, except at the narrow space near their proximal 

 ends. The pollex is represented by its metacarpal bone. The 

 femur has a third trochanter, with two patellas in front of, and two 

 fabellse behind, the condyles. I have found, also, a small ossifi- 

 cation at the anterior end of each semilunar cartilage. The 

 fibula is anchylosed to the tibia at both its extremities. The 

 entocuneiform is long, and applied to the inner side of the base 

 of the second metatarsal, but it supports a short metatarsal with 

 the first and ungual phalanx of its proper digit, the hallux. 



Rodents burrow chiefly for concealment, rarely for food : 

 the Rabbit needs but a slis^ht modification of the limbs, as com- 

 pared with the surface-dwelling Hare, to excavate, in loose soil, 

 its retreat. Perhaps the ' Mole-Rats ' of the Cape are the best 

 burrowers of the order. In Bathyergus the upper border of 

 the scapula describes an open angle ; its outer surface is nearly 

 equally bisected by the spine, which rises to an unusual height, 

 and sends ofl" a remarkably long subtrihedral acromion, the ex- 

 tremity of which appears as a thick epiphysis bent toward the 

 lon«: and strono^ clavicle with which it articulates. A well- 

 marked deltoid process stands out from the middle of the shaft 

 of the humerus, which is imperforate at its distal end. The 

 olecranon is unusually thick and expanded. The femur shows a 



