458 University of California Publications in Zoology [Vol. 13 



of the shaft of the bone is deeply grooved from the base of the 

 olecranon process to the junction of the middle and distal thirds. 

 The ulna ends in an enlarged head bearing the long, sharp styloid 

 process which articulates with the cuneiform bone {st. proc, figs. 

 H, I). 



The ulna in the various rodents examined is typical of the 

 whole skeleton of the limb — long and slender and curved dorso- 

 ventrad in the rabbits, relatively heavier in the tree squirrel, and 

 still heavier and more irregular in the ground squirrel and rat. 

 The most marked difference, however, is in the length of the 

 olecranon process. This difference is so significant that it is taken 

 up later in detail (see p. 487). 



The radius {rad., figs. H, J) is a fairly stout bone, three- 

 fourths the length of the ulna, and curved slightly with its con- 

 vexity cephalad. Its head is enlarged and bears an oval articular 

 surface which rests against the capitellum of the humerus. Distad 

 of the head the shaft is flattened dorso-ventrally for its upper third. 

 The distal two-thirds of the bone is somewhat angular, and increases 

 in diameter toward the extremity; this bears the sharp styloid 

 process mesially, and the articular surface for the scapholunar 

 laterally. Little rotary movement is provided for in the radius of 

 the gopher, as is shown by the very close attachment of the radius 

 and ulna in their distal half, and by the oval (not circular) prox- 

 imal articular surface of the radius. However, a slight amount of 

 supination and pronation is shown, even in the alcohol-hardened 

 ligamentary skeleton. 



The radius in the rabbits is much stronger than the ulna. In 

 the squirrels the bones are of about equal strength, while in the 

 rat and gopher the ulna is stronger, particularly in its proximal 

 half. The radius and ulna of the rabbits are very closely bound 

 together throughout their whole length, and probably permit of no 

 rotation whatever. In the tree squirrel the interosseous ligament is 

 broad, and the degree of rotation is undoubtedly greater than in 

 any of the other forms. Another evidence of this difference 

 between the rabbit and tree squirrel is the fact that in the squirrel 

 the bones of the forearm are practically straight, while in the 

 rabbits they are much curved. 



A remarkable structure connected with the skeleton of the 

 anterior leg of the gopher is the greatly enlarged and elongated 

 external collateral or lateral humero-ulnar ligament. In man, in 



