SUB-ORDER 1. MYSTACOCETI. This division is thus characterised:--Teeth are never functionally developed; they are present in the young, but replaced in the adult by the baleen or whalebone; the external respiratory aperture is double; the skull is perfectly symmetrical; the rami of the mandible are arched outwards and do not form a true symphysis; the sternum is always composed of a single piece of bone; the ribs articulate only with the transverse processes of the vertebrae. The Mystacoceti are nearly invariably huge creatures, the sole exceptions being the Pygmy Right Whale, _Neobalaena_, and {354} a small Rorqual. But even these are larger than the majority of Toothed Whales. The most characteristic feature by which the Whalebone Whales are to be distinguished from other Whales is that which gives to them their name, the presence of whalebone. Whalebone is a horny product of the epithelium lining the mouth, and is comparable to an exaggeration of the transverse ridges which are found in the mouths of all mammals upon the palate. In non-Cetacean mammals these ridges vary in depth, and are arranged as a rule transversely, but with an oblique inclination. This is precisely how the plates of baleen are disposed in the mouth of a Whale. Each piece of "bone" is triangular in shape, the broader end being that of attachment while it narrows gradually; the inner side of the blades is frayed out into a number of threads which form the straining apparatus. The plates vary in length up to as great an extreme length as 13 feet, which occurs in the Right Whale at times. The colour is black or paler, even white. The number of these plates in the mouth is very great. As many as 370 blades have been counted. They diminish in length towards both ends of the series. Though whalebone has been in use for a long period, whence the whalebone came was formerly one of those things not generally known. [Illustration] FIG. 189.--Section of upper jaw, with baleen plates, of _Balaenoptera_. _a_, Bone of jaw; _b_, gum; _c_, straight edge of baleen plate; _d_, _e_, frayed out surface of baleen plates. (After Owen.) A very prevalent notion was that the whalebone formed the eyelids or perhaps the eyelashes of the creature. Scaliger, commenting upon Aristotle, held that the whale had "lamellae upon the eyebrows, which, when the head is plunged below the surface, were raised by the water; but when the animal raised its head {355} above the waves the lamellae fell and covered the eyes." Whalebone, too, has been often spoken of as "the fin of a whale," "the finnes that stand forth of their mouths." The value of whalebone is still great, in spite of various substitutes which are now used in its place. In the year 1897, for example, the value of this article was L2000 per ton. As a single Whale may produce several tons of this material, it is not surprising to find that the results of a whaling voyage may be very profitable.