Aiu-. lo AXATOMY OF THE EARED AND EARLESS SEALS HOWELL 115 



(a rotator) is very weak while other muscles have encroached upon 

 the infraspinous space of the scapula, which is especially the case 

 in the phocid, for here this space is relatively considerably larger 

 than the supraspinous fossa. There is no very clear reason to be 

 seen for the extension caudad of the glenovertebral cartilage of the 

 Phoca, although it is self-evident that it has taken this course in 

 response to stimuli supplied by the muscles attached thereto. 



The percentage of arm length (humerus, radius and manus), 

 based on the bones only, to bod}^ length is in a cat skeleton 82, 

 Zalophus 66, and Phoca 48 per cent, so it is seen that in compari- 

 son with a fissiped the phocid arm is much reduced, while that of 

 t]\e otariid occupies an intermediate position. In tiie same order 

 as aboA^e, the length of the humerus compared to body length is, 

 respectively, 31, 18, and 14 per cent; of the radius, 30, 20, and 14 

 per cent; and of the bony part of the manus, 21, 29, and 20 per cent. 

 It is thus seen that there has been a shortening of the two upper 

 segments of the pinnijjed arm and that the proportions of one seg- 

 ment to the other have remained almost the same as in the fissiped, 

 save possibly that the rate of reduction in the size of the humerus 

 lia^ been a bit more rapid in the case of Zalopkus. There has been 

 no change in the size of the manus of Phoca relative to the entire 

 arm, but that of the Zalophus has increased in relative size (com- 

 l^ared to the more proximal segments) about one-third. Prasumably, 

 however, this osteological increase in the size of the otariid manus has 

 not been sufficiently rapid to meet the needs of the animal and a still 

 larger area for furnishing propulsive force has been acquired by the 

 development of cartilagenous extensions to the digits. As an alter- 

 native one must consider the unlikely possibilit}'^ that the presence of 

 these cartilages is due to some stimulus other than that caused by 

 the need of a longer manus in swimming. At any rate, they have 

 made the functional length of the manus (measured to its tip) of the 

 Zalophits about 40 per cent of the body length, or, from a relative 

 standpoint, fully twice as long as in the Phoca. From still another 

 aspect, the effective length of the anterior limb operating against 

 the water is the distance from the axilla to the tip of the flipper. In 

 the Zalophiis with the axilla at a point just proximad to the middle 

 of the ulna, this amounts to 52 per cent of the trunk length. In 

 the Phoca., the axilla is opposite the ulnare and the corresponding 

 percentage is about 19. As the anterior limb is used for such very 

 different purposes in the two families, however, a comparison of the 

 visible portions does little but call attention to their dissimilarities. 



In the Otariidae the extended arm may be operated to good 

 advantage as a swimming organ by muscles coming from other parts 

 of the body. Thus the cephalohumeral is attached in part to the 



