ART. 15 ANATOMY OF THE EARED AND EARLESS SEALS HOWELL 29 



8. Except in shortness, the only way in wiiich the two humeri show 

 a marked resemblance is in the enormus development of the deltoid 

 crest. 



In the Zalophus the greater tuberosity is very massive and rises 

 well above the head in an irregularly rounded knob, and distad it is 

 prolonged into the broad deltoid crest, which extends for more than 

 half of the shaft, and is raised from the shaft by a relatively thin 

 wall of bone, which in the Phoca is thicker because of the lesser depth 

 in that animal of the fossa from which arises the brachialis. In 

 the Phoca the greater tuberosity is no higher than the head and the 

 part adjoining the latter is relatively narrow, while there is a pro- 

 jection of bone stretching toward the lessor tuberosity, between which 

 there extends a stout ligament as in many carnivores. The deltoid 

 crest is relatively broader than in Zalophus — almost as broad as the 

 shaft — the borders are more overhanging, and the distal termination 

 extends no farther than the middle of the shaft. Upon the lateral 

 border is a pit, of varying definition individually, for the reception 

 of a separate tendon of the humerotrapezius. The only striking dif- 

 ference in muscle insertions that might account for the variation of 

 the greater tuberosity is the much larger supraspinatus in Zalovhus^ 

 but the chief reason is undoubtedly the different sort of work, and 

 at a different angle, that the muscles of the two animals need to 

 perform. The greater height in the otariicl would allow of more 

 powerful extension of the humerus. And the farther extension 

 distad of the deltoid crest in this animal provides a greater leverage 

 for the deltoid and pectoralis. In the Zalophus the lesser tuberosity, 

 placed mediocraniacl of the head, is very much lower than the head, 

 but is massively rounded and its base gradually merges with the 

 shaft. In the Phoca the homologue of the lesser tuberosity is 

 phenominally developed, somewhat falciform, and higher than either 

 the head or the " greater " tuberosity. Its base assumes more the 

 form of a ridge. To account for these differences there is ample 

 muscular variation. The subscapularis is of course inserted upon 

 this tuberosity and it is evidently much better developed and hence 

 more powerful in the phocid. In this animal a portion of the 

 cephalo-humeral and the ligament between the tuberosities probably 

 account for the falciform part of the lesser, while there is also inser- 

 tion of the small subscapulo-capsularis, absent in the otariid, but the 

 latter has the episubscapularis, which the phocid lacks. In final 

 analysis, however, it is impossible to say that the diametrically 

 opposed conditions of the two tuberosities in these two animals are 

 due to this or that muscle. 



The bicipital groove lies between the two tuberosities and this is 

 relatively much the more capacious in Phoca. Within its channel, 

 just proximad to the middle of the shaft, lies the teres major rugosity, 



