AQUATIC MAMMALS 



least some of the Mysticeti the stimulus for broadening of the scapula 

 has resulted in an extreme development at the glenovertebral, and to a 

 lesser extent at the coracovertebral, angle of the suprascapular cartilage 

 (figure 36). The chief reason for this extreme cartilaginous extension 

 in a posterior direction is seen in figures given by Schulte (1916) to be 

 probably the serratus magnus muscle, attached to the posterior part. This 

 muscle is thereby given an especially efficient lever arm for operating as a 

 depresser anguli scapulae, ostensibly of great use in tilting the flipper for 

 equilibration. It should also be mentioned that in Mysticeti the deltoid 

 has become differentiated both in origin and insertion for assuming in 

 even fuller degree the function normally held by the supraspinatus. 



In the Cetacea the condition of the acromion is quite curious and there 

 is apparently no good reason for its existence. It does not increase the 

 leverage of any muscle, for nothing is attached to it save incidentally. 

 The same, may be said of the coracoid process. Both may be developed 

 to an extraordinary degree, as in Sibbaldus for instance, or both may be 

 entirely obliterated, as in Megaptera (a southern species of this genus 

 is said to have a short acromion), apparently with an equal lack of 

 reason for both conditions. 



It is interesting to note that the scapula in the zeuglodont Basilosaurus 

 is exactly what one would expect it to be supposing that it represents a 

 stage through which modern whales have passed. In shape it is entirely 

 whale-like. The supraspinous fossa is relatively smaller than usual but 

 still much larger than in any whale, being about a third or two-fifths 

 of the infraspinous fossa. Along the glenoid border there is also a 

 fossa for a strong teres muscle. The acromion is greatly developed. 



To a large extent scapular conditions in the sirenians resemble those 

 in the pinnipeds, but Murie did not give sufficient detail regarding this 

 part of the manati to make it advisable for me to pursue the subject fur- 

 ther. Noteworthy in the manati is the irregularity of the spine, appar- 

 ently attributable to some detail of the deltoid, and the pointed acromion. 

 In the dugong the acromion is strongly distinct but it projects at a right 

 angle to the scapular plane. 



Ostensibly the medial muscles of the sea-lion scapula must be those 

 which have been instrumental in causing the broadening of this bone, 

 for the lateral ones are very different from those in whales. The supras- 

 pinous fossa is considerably larger than the infraspinous space, while 

 in the Phocidae it is smaller. In both groups the origin of the infraspin- 

 ous muscle occupies about half the space posterior to the spine, but the 

 muscular conditions over the remainder of this area are so involved and 



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