486 University of California Publications in Geology [Vol.7 



These modifications are indicative of adaptation to life in 

 the water, in that an animal living in the water does not depend 

 so much upon its fore limbs as upon flotation for support. This 

 lack of need of a supporting function allows a weakening to take 

 place first in the muscles of the fore limbs, and then in the pro- 

 cesses for their attachment. J. C. Merriam 30 found the scapula 

 in the later Jurassic ichthyosaurs to be narrowed distally, and 

 suggests that this character may be ascribed to the effect of 

 aquatic adaptation. 



Ribs 



The ribs are heavy as compared with those of Lutra. This 

 may be an adaptation to life in the water, since it may be desir- 

 able for the ribs to have increased weight so as better to resist 

 the pressure of the denser medium. 



Pelvis 



The pelvis is elevated somewhat so as to lie more nearly 

 parallel to the vertebral column than in the river otter. The ilia 

 are very markedly turned outward anteriorly. Their superior 

 borders or crests serve as places of attachment, at least partially, 

 for the following muscles of the hind limb : the gluteus medius, 

 gluteus minimus, and sartorius. The sartorius is the largest 

 and probably the most important of these muscles. Its action 

 is the adduction and rotation of the femur and the extension 

 of the tibia. It is noteworthy that the proximal portion of 

 the crest of the tibia, to which the sartorius is fastened, is more 

 roughened in the sea-otter than in the river otter. The pubic 

 bones are not (in no. 6956) joined along the symphyseal line, 

 except by cartilage, but they would possibly become somewhat 

 more intimately united as the age of the animal increased. The 

 Stanford University specimen, although a middle-aged adult, still 

 shows the pubic bones not ankylosed. This fact would indicate 

 that this is the normal condition in Latax. 



In the seal the pelvis is approximately parallel to the ver- 

 tebral column, the ilia flare outward anteriorly, so as to be nearly 



3u Merriam, J. C, " Triassic ichthyosauria, etc.," Mem. Univ. Calif., 

 vol. 1 (1908), p. 75. 



