AQUATIC MAMMALS 



although it is occasionally present in Otariidae and possibly more rarely 

 in the Phocidae, and is imperforate. Aquatic modification seems appar- 

 ent in the interorbital region of otters and pinnipeds. The interorbital 

 septum is thinner in the former than in most fissipeds, is 11 per cent of 

 the skull length in Zalophus and may be as little as 3 per cent in Phoca, 

 in which it is so reduced that the ethmoturbinals have forced their way 

 to the surface of the bone. This reduction in the interorbital width is 

 ostensibly for the purpose of permitting more dorsal vision, best ac- 

 complished by reducing this width so that the eyes may roll farther 

 mediad and hence dorsad. The size of the orbits in the Phocidae are 

 usually larger than in the Otariidae to accommodate the larger eyes of 

 the former, and this has had a tendency to force the postorbital processes 

 of the zygomatic arches farther caudad. The temporal fossae of the 

 Otariidae indicate temporal muscles which are much more powerful 

 than one would expect to find in a fish-eating mammal — an anatomical 

 arrangement which is also the case with the masseter muscles — and these 

 conditions are reflected in the rather robust mandible. The skull of 

 the otter is also well muscled and that of some of the Pocidae, but 

 the tendency in most of the latter is for a reduction in the size of the 

 temporal fossae, which may not meet the sagittal line; and correspond- 

 ing weakening of the mandible. 



No trustworthy conclusions may be drawn anent the palatal and 

 pterygoid regions, nor regarding the bullae, for these may or may not re- 

 flect conditions in terrestrial ancestors. Although no more marked than in 

 many terrestrial forms, the occipital condyles in Zalophus, for instance, 

 are rather narrow, for this animal has occasion to twist its head in all 

 directions — movements which are facilitated by a circumscribed joint. 

 In Phoca the condyles are somewhat wider, for the diflferent method of 

 swimming which it employs militates again great mobility of the head 

 during progression, and broader condyles make a stronger joint. 



It is in the occipital region that the pinniped skull shows the greatest 

 results of the aquatic stimulus. In the usual fissiped, as in the otter, 

 the occipital plane is either practically certical or else slopes rearward 

 while in Zalophus the slope is definitely forward, and still more so in 

 Phoca. The reason for this is discussed under the Cetacea. In Zalophus 

 the lambdoidal crest bordering the occipital plane is strongly developed, 

 for strong muscles (cephalohumeral, sternomastoid, trachelomastoid, 

 rhomboideus anticus, splenius and semispinalis capitis) which, as a 

 whole, extend uninterruptedly from the vertex to the mastoid process. 

 In Phoca there is no crest formed in the lambdoid region, and the semi- 



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