In the skeleton it is to the amount of cartilage between the bones, along with the gristly rods attaching the ribs to the back and breast-bones, that the extraordinary mobility of figure on land, and easy motions of swimming in the water, which belong _par excellence_ to the Marine Carnivora, are due. Add to this that the hip-bones are narrow and remarkably compressed, the thigh-bones excessively short, the shank-bones long and tied in behind, while great hind-flippers, like double oars rearwards, drive or steer with sculling sweep. The bones of the fore-limb and its modified foot altogether are strong, and remarkably so in the powerful-swimming Sea Lions. All four feet have excessively long toes, the thumb-bones being longest, the fingers lessening to the little toes; in the hind-foot the three middle toes are shorter than the two outer ones. There are tiny nails on each toe at the bone ends, beyond which is a flat spatula-shaped cartilage, of excessive length in the Otary family. The webbed flat feet are thus altogether very peculiar, and when used the entire sole, even including wrist and ankle-bones, is laid flat on the ground, so that two families of the Pinnipedia are really more plantigrade than the Bears. The Common Seals, or Phocidæ, however, never use the hind-feet on land, and the fore-feet but sparingly, while their nails are more claw-like than in their marine congeners. In none of the Seal tribe, though, are the nails or claws retractile, as previously has been shown (p. 12) in the Cat and Lion.