112 PEOCEEDINGS OF THE NATIONAL MUSEUM vol.73 



propels itself through the water at speed by means of its posterior 

 parts can wave its head around in all directions, but an otariid, with 

 center of motion near the middle of its mass, can move the head in 

 any direction it pleases if it at the same time employ its rear end in 

 antagonistic or compensating movements. 



Accompanying this state of affairs is found a distinctive condition 

 of the muscles with attachment to the occipital crest and ventrad. 

 In contrast to the situation in Zalophus^ in the Phoca the cranial 

 attachments of the cephalohumeral, humerotrapezius, and anterior 

 rhomboid are narrowly confined to the vicinity of the vertex, while 

 the middle and ventral parts of the occipital crest are free of all 

 muscles which are more characteristic of the normal occipital crest, 

 the superior oblique being the only one found here. As the above 

 have narrowed mediad, so the others have narrowed ventrad, and 

 the insertions of the sternomastoid, trachelomastoid, and splenius are 

 confined to the mastoid process. Thus the muscles here discussed 

 are better situated to work in two planes of movement in the Phoca — 

 directly dorsad and directly laterad — which are most effective in 

 rudder movements, rather than muscles fitted for movement in all 

 directions, as in the otariid. 



The number of vertebrae in the thoracico-lumbar series, 20, is the 

 same as in the majority of fissipeds, which is one more than the prim- 

 itive number as mentioned by Todd (1922), while the tendency in 

 most orders is toward an increase in the number of this series. The 

 almost total lack of definition of the neural spines in the anterior 

 thoracic region of Phoca conforms to the claim by Harrison Allen 

 (1888) that these spines are practically absent in those mammals 

 which do not use the anterior limbs for support, such as Dipus and 

 the bats. This statement furnishes food for thought, but the con- 

 ditions are far from being as simple as implied, for in the Cetacea, 

 which have surely abandoned ' manual support for a longer time 

 than any other mammal, the neural spines of the anterior thorax 

 are always well developed and at times are enormous. It might be 

 thought that in view of the greater development of the back muscles 

 in the Phocidae the spines might be higher than in the Otariidae, 

 rather than so much lower, but such is not the case, and it is probable 

 that the narrow, deeper back musculature of Zalophus, with the 

 longer spines, has developed for movements of all sorts, especially in 

 the sagittal plane, while the immensely broad back musculature of 

 Phoca^ with wide vertebral articulations and very low spines is in 

 response to movements that have been so largely lateral. Photo- 

 graphs of Mirounga (see fig. 29) show that from a prone position 

 on land this animal can elevate the anterior part of the body, includ- 

 ing most of the thorax, to an absolutely vertical position in a manner 

 that one would judge to be beyond the power of any mammal. There 



