394 MAMMALIA [GET ACE A. position of the narial aperture and the length and flatness of the nasal bones, which distinguish them from all existing forms, we must also suppose to be a character at one time common to all Cetaceans, though uow retained (but to a less degree) only by the Mystacocetes. Even Squalodon, which in its heterodont dentition so much resembles Zeuglodon as to have been placed by some zoologists in the same genus, entirely differs from it, and conforms with the ordinary Dolphins in its essential cranial characters. The origin of the Cetacea is at present involved in much obscurity. They present no signs of closer affinity to any of the lower classes of vertebrates than do many other members of their own class. Indeed in all that essentially distinguishes a mammal from the oviparous vertebrates, whether in the osseous, nervous, reproductive, or any other system, they are as truly mammalian as any other group. Any supposed marks of inferiority, as absence of limb structure, of hairy covering, of lacrymal apparatus, &c., are obviously modifications (or degradations, as they may be termed) in adaptation to their special mode of life. The characters of the teeth of Zeuylodon and other extinct forms, and also . of the foetal Mystacocetes, clearly indicate that they have been derived from mammals in which the heterodont type of dentition was fully established. The steps by which a land mammal may have been modified into a purely aquatic one are clearly indicated by the stages which still survive among the Carnivora, in the Otarix, and in the true Seals. A further change in the same direction would produce an animal somewhat resembling a Dolphin, and it has been thought that this may have been the route by which the Cetacean form has been developed. There are, however, great difficulties in the way of this view. If the hind limbs had ever been developed into the very efficient aquatic propelling organs they present in the Seals, it is not easy to imagine how they could have become completely atrophied and their function transferred to the tail. It is more likely that the Whales were derived from animals with long tails, which were used in swimming, eventually with such effect that the hind limbs became no longer necessary. The powerful tail, with its lateral cutaneous flanges, of an American species of Otter (Pteronura sandbachii) may give au idea of this member in the primitive Cetaceans. But the structure of the Cetacea is, in so many essential characters, so unlike that of the Carnivora that the probabilities are against these orders being nearly related. Even in the skull of the Zeuglodon, which has been cited as presenting a great resemblance to that of a Seal, quite as many likenesses may be traced to one of the primitive Pig-like Ungulates (except in the purely adaptive character of the form of the teeth), while the elongated larynx, 1 complex stomach, simple liver, reproductive organs both male and female, and foetal membranes of the existing Cetacea are far more like those of that group than of the Carnivora. Indeed it appears probable that the old popular idea which affixed the name of " Sea-Hog " 2 to the Porpoise contains a larger element of truth than the speculations of many accomplished zoologists of modern times. The fact that Ptatanista, which, as mentioned above, appears to retain more of the primitive characteristics of the group than any other existing form, and also the somewhat related Inia from South America, are both to the present day exclusively fluviatile, may point to the freshwater origin of the whole group, in which case their otherwise rather inexplicable absence from the seas of the Cretaceous period would be accounted for. 1 There is much resemblance in the larynx of the Hippopotamus, but none in that of the Seal, to the same organ in the Cetacea. 2 German, Mcerschwein, whence the French Marsouin. "Porpoise" is said to be derived from " Porc-poisson." SUBOUDER MYSTACOCETI, the BAL^EXOIDKA, or Whalebone Whales. Teeth never functionally developed, but always disappearing before the close of intra-uterine life. Palate provided with plates- of baleen or " whalebone." Skull symmetrical. Nasal bones form ing a roof to the anterior nasal passages, which are directed upwards and forwards. Maxilla produced in front of, but not over, the- orbital process of the frontal. Lacrymal bones small and distinct from the jugal. Tympanic bone ankylosed with the periotic, which is attached to the base of the cranium by two strong diverging processes. Olfactory organ distinctly developed. Kami of mandible arched outwards, their anterior ends meeting at air angle, and connected by iibrous tissue without any true symphysis. All the ribs at their upper extremity articulating only with the transverse processes of the vertebra ; their capitular processes when present not articulating directly with the bodies of the vertebne. Sternum composed of a single piece, and articulating only with a single pair of ribs. No ossified sternal ribs. External openings of nostrils distinct from each other, longitudinal. A short conical cte.cum. These animals have, when in the fcetal state, numerous, minute, calcified teeth lying in the dental groove of both upper and lower jaws. They are best developed about the middle of fuetal life, after which period they are absorbed, and no trace of them remains at the time of birth. 3 The baleen or whalebone does not make its appear ance until after birth. It consists of a series of flattened horny plates, between three and four hundred in number, on each side of the palate, with a bare interval along the middle line. They are placed transversely to the long axis of the palate, with very short intervals between them. Each plate or blade is somewhat triangular in form, with the base attached to the palate and the apex hanging downwards. The outer edge of the blade is hard and smooth, but the inner edge and apex fray out into long bristly fibres, so that the roof of the whale s mouth looks as if covered with hair, as described by Aristotle. At the inner edge of each principal blade are two or three much smaller or subsidiary blades. The principal blades are longest near the middle of the series, and gradually diminish towards the front and back of the mouth. The horny plates grow from a dense fibrous and highly vascular matrix, which covers the palatal surface of the maxilla?, and which sends out lamellar processes, one of which penetrates the base of each blade. More over, the free edge of these processes is covered with very long vascular thread-like papillse, one of which forms the central axis of each of the hair-like epidermic fibres of which the blade is mainly composed. A tranverse section of fresh whalebone shows that it is made up of numbers of these soft vascular papilhe, circular in outline, each surrounded by concentrically arranged epidermic cells, the whole bound together by other epidermic cells, that constitute the smooth cortical (so-called enamel ") surface of the blade, which, disintegrating at the free edge, allows the individual fibres to become loose and assume the hair-like appearance before spoken of. These fibres differ from hairs in not being formed in depressed follicles in the enderon, but rather resemble the fibres compos ing the horn of the Rhinoceros. The whalebone in fact consists of nothing more than modified papillae of the buccal mucous membrane, with an excessive and cornified epithelial development. The blades are supported, and bound together for a certain distance from their base, by a mass of less hardened epithelium, secreted by the surface of the palatal membrane or matrix of the whale bone in the intervals of the lamellar processes. This is the " inter mediate substance" of Hunter, the "gum" of the whalers. Baleen varies much in colour in different species. In some it is almost jet black, in others slate colour, horn colour, yellow, or even creamy-white. In some the blades are variegated with longitudinal stripes of different hues. It differs also greatly in other respects, being short, thick, coarse, and stiff in some, and greatly elongated and highly elastic in those species in which it has attained its fullest development. Its function is to strain the vrater from the small marine molluscs, crustaceans, or fish upon which the whales subsist. In feeding they fill the immense mouth with water containing shoals of these small creatures, and then, on their closing the jaws and raising the tongue, so as to diminish the cavity of the mouth, the water streams out through the narrow intervals between the hairy fringe of the whalebone blades, and escapes through the lips, leaving the living prey to be swallowed. 4 Our knowledge of the different structural modifications at- 3 These were discovered in the Greenland Whale by Geoffroy St Hilaire, whose observations were confirmed and extended to other genera by Eschricht. They have lately been very fully described in Balxnoptera rostrata by Julin (Archives de Biologic, i., 1880). 4 For the structure of whalebone, see Hunter, Observations on the Structure and Economy of Whales," Phil. Trans., 1787 ; Eschricht and R< inhardt, On the Greenland Right Whale, English translation by the Ray Society, 1866, pp. 67-78 ; and W. Turner, in Traits. Roy.
Soc. Edin., 1870.