116 PEOCEEDIlSrGS OF THE NATIONAL MUSEUM vol. 7& 



deltoid crest but it also has connection with the tissue about the 

 presternum and the border of the pectoralis. Its chief action is 

 upon the head and neck but it also extends the humerus both in 

 swimming, and upon the land by means of lunging, forward and 

 back movements of the head, and is thus of definite aid in terrestrial 

 progression. It is also involved with action of the anterior part 

 of the pectoralis, which by means of its insertion as far distad actu- 

 ally as the palmar tissue, adducts the arm with much power. Of 

 the greatest importance in strong backward sweeps of the flipper 

 are the intimately connected posterior portions of the pectoralis, the 

 latissimus dorsi, and most of the panniculus carnosus, all of which 

 have a long power arm upon the anterior extremity. In the Phoca 

 these muscles perform other work and will be discussed elsewhere. 



Relative to length of body the humerus of Zalophus is less than 

 65, and of the PJwca} about 45 per cent of the length of this bone 

 in the cat. It is proportionately massive, with prominent and broad 

 processes and ridges. Both of these details denote great power of 

 the muscles attached. The humeral head differs somewhat in the 

 two animals, indicating that the normal position of the humerus in 

 Phoca is slightly more flexed and abducted than in Zalophus. Ex- 

 tension and flexion of the humerus is limited largely by the fact that 

 it is entirely within the body covering and that the integument limits 

 the movements of the forearm also; but abduction of the humerus 

 is fully as great as in many fissipeds — Cards for instance. As the 

 arm is largely within the body, flexion of the part distad of the 

 humerus, by the single biceps and brachialis, is limited, while exten- 

 sion of the humerus is well provided for by a complex triceps with a 

 leverage much greater than in the normal fissiped. 



In the otariid the great height and massiveness of the greater 

 tuberosity is for supplying increased leverage to the large supraspina- 

 tus which helps extend the humerus — a motion of much importance 

 in recovery after a backward sweep of the foreflipper. This move- 

 ment is not of importance to the Phoca^ so the supraspinatus is small 

 and the greater tuberosity lower than the femoral head. The enor- 

 mous development of the deltoid crest, extending distad from the 

 greater tuberosity for tAvo-thirds the distance to the condyle in 

 Zalophus and half the distance in Phoca., is of the utmost importance 

 in not only supplying several times the leverage to the shortened 

 bone that these muscles could furnish to a humerus of the human type» 

 but also, through the great elevation of the deltoid crest, in presenting 

 an efficient lever arm for strong rotation, needed in the " feather- 

 ing " action of the flipper of Zalophus in swimming. The cephalo- 

 humeral, humerotrapezius, and pectoralis have probably had far more 

 to do with the elevation of this crest than the deltoideus. In the 

 Phoca the deltoid crest, although not quite so long is fully as high — 



