The whales, as compared with their dog-like ancestors, are modified to a more extreme degree and in more special ways than is the case in any other group of which we can trace the history over a similar period of development. This is connected with the complete change of conditions of life to which these mammals ("warm-blooded, air-breathing quadrupeds which suckle their young") have become adapted in passing from a terrestrial to a marine existence. Other mammalian ancestors have independently taken to a marine life and given rise to strange-looking adaptations, namely, the seals and also the Manatee and Dugong known as the Sirenians (so-called because they give rise to sailors' stories of mermaids and sirens), but these are far less changed, less modified than the whales. The whales have acquired a completely fish-like form. They frequently have a large back fin, and have lost the hind legs altogether. The horizontally spread flukes of the whale's tail have nothing to do with the hind legs, whereas the common seal's hind legs are tied together so as to form a sort of tail. In the bigger whales, sunk deep in the muscle and blubber, we find on each side well forward in the body (not near the tail) a pair of isolated, unattached bony pieces, which are the hip-bone and thigh-bone--all that remains of the hind limbs. The neck is so short that in many whales the seven neck-bones, or "vertebrae," are all fused into one solid piece not longer than a single ordinary vertebra, and showing six grooves marking off the seven vertebrae which have united into one. The head is more strangely altered than any other part of the whale. The jaws are greatly elongated--so as to give a beak-like form in all--but this region is specially long and narrow in the "beaked whales" known to zoologists by the name Ziphius, in which it consists of a solid piece of ivory-like bone, which we find in a fossil state in the bone-bed of the Suffolk Crag. Farther back the bones of the face are suddenly widened in all whales and porpoises, and in many these bones grow up into enormous crests and ridges. The nostrils, instead of being placed, as in other animals, at the free end of the snout or beak, lie far back, so as to form the "blow-hole," which is near the middle of the head.