456 University of California Publications in Zoology [Vol. 13 



The scapula iu the rabbits, the squirrels and the rat is rela- 

 tively longer and more slender than in the gopher, and the eoracoid 

 process is short, especially in the rabbits. The spinous process, 

 however, shows most variation in the different types. In the 

 rabbits the spine is low and its caudal surface is concave. It ends 

 in a short acromion, while the metacromion forms a long process 

 extending backward from the acromion at right angles to the spine. 

 In the squirrels the acromion is the longer process, and curves 

 eephalad from the tip of the spine. In the rat the end of the spine 

 is flattened laterally and the acromion and metacromion are not 

 distinct. 



The humerus (figs. F, G) in the gopher is a short, irregular 

 bone with enlarged extremities and a high process near the middle 

 of the cephalo-lateral border. The proximal extremity is composed 

 of three enlargments; the head, the greater tuberosity laterad, and 

 the lesser tuberosity mesiad. Between the two tuberosities on the 

 cephalic surface is the bicipital groove. The shaft just distal to 

 the head is triangular, with its sharpest angle, formed by the deltoid 

 ridge, cephalo-laterad. The deltoid ridge begins at the base of the 

 greater tuberosity and increases rapidly in height until about the 

 middle of the bone, where it forms the high deltoid tuberosity. 

 This tuberosity is one of the characteristics of the humerus of the 

 gopher; it is as high as the whole diameter of the bone just distal 

 to it (fig. F). Just below this point the bone is flattened laterally, 

 but it rapidly broadens out so that it becomes flat antero-pos- 

 teriorly, spreading to form the high condyles. The external supra- 

 condylar ridge is high and sharp; the internal supracondylar ridge 

 is low and rounded, but this condyle is much higher than the 

 external. The articular surface occupies about two-thirds of the 

 width of the distal end of the bone. This articulation is an excel- 

 lent example of a hinge joint without any lateral motion. Examina- 

 tion of the bones shows that the forearm is incapable of being set 

 in a straight line with the upper arm; at the greatest extension 

 the bones form an angle of about 135°. 



Of the other types examined the humerus of the rat most 

 nearly resembles that of the gopher. It is irregular in the former, 

 but relatively more slender than in the gopher. The humeri of the 

 two squirrels are much alike in shape, with the deltoid tuberosities 

 low and rounded, and the external supracondylar ridges remarkably 

 long and high. The humerus of the ground squirrel is proportion- 



