ART. 15 AXATOSIY OF THE EARED AND EARLESS SEALS HOWELL 12 1 



together with mechanisms closing the nostrils and ears, is a funda- 

 mental attribute which almost all aquatic animals develop at a 

 relatively early stage in their aquatic evolution. The nails of the 

 manus in Zdlophus have deteriorated until they exist merely as 

 horny spots within integumentary^ pits. As the animal has made 

 no use of them, for scratching itself, as weapons of offense, or as 

 tools to aid in the capture of food, they have naturally atrophied. 

 The nails of Phoca are large and well formed. The external por- 

 tion of the anterior limb is too short for these to be used for scratch- 

 ing any considerable area of the body, and even if for this use 

 exclusively they would doubtless have become slender; and so far 

 as is known i\\ey are of no use in the securing of food. Thus their 

 use for scratching holes through the ice, as claimed by sealers, is the 

 most likely theory to account for their robust development. 



In the otariid the interdependence of the different parts of the pos- 

 terior half of the body during terrestrial locomotion is extremely 

 close, and in the phocid this rela- 

 tionship is even closer, but during 

 the act of swimming only. It will 

 be recalled that the hind limbs of 

 the Otariidae are used during 

 movement on land in a planti- 

 grade manner by causing the axis 

 of the sacral vertebrae to assume a 

 vertical position, while the hind 

 limbs are not of primary impor- 



x„ -\ • 1 ,• „iJ.ix Fig. 29. — Position assumed by Mirodnga 



tance durmg natation; and that ilt.ustr..tikg possible degree of ver- 



the Phocidae move upon land ex- tebral bending (after a PHOTooRAPn 



,1 ,1 1 1 1 1 ii IN THE Illustrated London News of 



actly as they would had they no ^^^^ ^^ ^327) 

 limbs at all, while progression 



through the water is solely by means of oscillating lateral movements 

 of the rear end. In the otariid the segments of the hind limb proxi- 

 mad of the heel are used upon land only as immobile supports, for 

 they are too closely bound down to the innominates for much inde- 

 pendent movement. Their terrestrial function therefore depends 

 solely upon the great elasticity of the lumbar region, through the 

 intervertebral disks. The inability of the Phocidae to use their hind 

 limbs on land depends upon several conditions, one of which seems to 

 be inability of the lumbar region to bend ventrad to the same degree 

 as in the eared seals; but yet the vertebral column of the phocids is 

 very elastic in certain directions, and Mirounga at least may stretch 

 dorsad so that the backbone is bent into a veritable right angle and 

 a rather sharp one at that. (See fig. 29.) 



The long back muscles of Zalophus are essentially similar to those 

 of a fissiped, and the hypaxial musculature is also unspecialized in 



