The genus is known typically by _H. rostratus_ (Fig. 84), but an imperfect skull has been made the type of _H. planifrons_—a species differing considerably in cranial characters from the typical one. The females and young males of the first-named species have the contour of the head of the same general form as in Fig. 84; the premaxillary crests of the cranium being widely separated from one another, and terminating in comparatively sharp edges. In the males, however, as age advances the summits of these crests become gradually expanded and flattened, till they are almost or quite in contact in the middle line. This development of the maxillary crests produces a corresponding elevation and flattening of the front of the head, so that in very old males this aspect presents a flattened disc-like surface rising abruptly from the beak (which thus becomes almost buried) and situated in a plane nearly at right angles to the line of the back.[145] So different, indeed, is the appearance of the skull of an old male from that of a female individual that it was long considered that they belonged to different species—the male form having been described as _H. latifrons_. The length of an adult male reaches 30 feet, while that of the female does not exceed 24 feet. The Hyperoödon, sometimes called “Bottlenose,” a name also vaguely given to several species of Dolphin, is a regular inhabitant of the North Atlantic, passing the summer in the Spitzbergen seas and going farther south in winter. It resembles the Sperm Whale in possessing a large store of oil in the upper part of the head, which yields spermaceti when refined; on this account, and also for the sake of the blubber, which supplies an oil almost indistinguishable from sperm-oil, this Whale has been the object of a regular chase in recent years. The following account of its habits is taken from a paper by Captain D. Gray, published in the _Zoological Society’s Proceedings_ for 1882:—