AQUATIC MAMMALS 



in the seals (Phocidae) . In certain respects the swimming of Potomogale 

 and the Phocidae is similar. In neither are the fore limbs used in swim- 

 ming, and in both the hinder end is swung from side to side. Therefore, 

 as discussed more fully in the case of the seal, there is a tendency for 

 the posterior part of the pectoralis and the latissimus dorsi to develop 

 toward the end that they may assist in pulling the posterior part to the 

 side. In Potomogale there is a clear tendency in this direction, and the 

 two muscles are continuous along their borders. The anterior part of 

 the pectoralis unites over the arm with the inferior margin of the trape- 

 zius, insertion being upon the humerus distad of the greater tuberosity. 

 By means of these two muscles, at least, the arm may easily be held sta- 

 tionery, acting as a base from which other muscles may operate. De- 

 serving of mention in this connection is also the sternocleidomastoid, 

 which is apparently of much importance in lateral movements of the head 

 that are requisite to swimming. This muscle arises from the midventrat 

 line for a surprising distance beneath the pectoral mass. 



In mounted skeletons of seals the anterior thorax is usually slightly 

 broader than high, or it may be markedly broader (P. groenlandica) ; in 

 sea-lions it is definitely narrowed ; and in the walrus fairly intermediate 

 between the two. These facts need not be of great significance in the 

 present connection, however, for there is equal variation in terrestrial 

 carnivores. At any rate this difference in pinnipeds is accentuated, in 

 the case of preserved specimens, to the point where a seal can hardly 

 be balanced upon its side, while one has difficulty in so balancing a sea- 

 lion upon its sternum. The sea-lion spends considerable time on land, 

 supported partly by the anterior limbs. The thorax thus experiences a 

 ventral gravitational pull for at least some of the time, and during this 

 time there is the same static muscle stress as acts upon the usual terres- 

 trial quadruped. High spinous processes and accompanying muscula- 

 ture in this region further accentuate the depth of the trunk as compared 

 to its width. What effect the levitating influence of the inflated lungs or 

 of the swimming actions of the anterior limbs have had while the animal 

 is in the water we do not know, although it is not improbable that the 

 constant adductive movements of the flippers have tended to make the 

 chest narrower. 



At least most seals of the genus Phoca never use the anterior limb 

 on land for supporting the body save for the briefest periods, although 

 some others of the Phocidae, as Mirounga, habitually do so. Further- 

 more, gravitational force can very rarely act upon the sternum. On the 

 contrary the sternum presses upon the ground both when the animal is 



[156] 



