June 28, 1883] 



NA TURK 



201 



mammalia. With their adaptation to an aquatic mode 

 of existence, organs fitted only for smelling in air became 

 useless, and so have dwindled or completely disappeared. 

 Time and circumstances have not permitted the acquisi- 

 tion of anything analogous to the special aquatic smelling 

 apparatus of fishes, the result being that whales are prac- 

 tically deprived of whatever advantage this sense may be 

 to other animals. 



It is characteristic of the greater number of mammalia 

 to have their jaws furnished with teeth having a definite 

 structure and mode of development. In all the most 

 typical forms these teeth are limited in number, not ex- 

 ceeding eleven on each side of each jaw, or forty-four in 

 all, and are differentiated in shape in different parts of 

 the series, being more simple in front, broader and more 

 complex behind. Such a dentition is described as "hete- 

 rodont.'' In most cases also there are two distinct sets 

 of teeth during the lifetime of the animal, constituting a 

 condition technically called "diphyodont." 



All the Cetacea present some traces of teeth, which in 

 structure and mode of development resemble those of 

 mammals, and not those of the lower vertebrated classes, 

 but they are always found in a more or less imperfect 

 state. In the first place, at all events in existing species, 

 they are never truly heterodmt, all the teeth of the series 

 resembling each other more or less or belonging to the 

 condition called " homodont," and not obeying the usual 

 numerical rule, often falling short of, but in many cases 

 greatly exceeding it. The most typical Odontocetes, or 

 toothed whales, have a large number of similar, simple, 

 conical, recurved, pointed teeth, alike on both sides and 

 in the upper and under jaws, admirably adapted for catch- 

 ing slippery, living prey, such as fish, which are swal- 

 lowed whole without mastication. In one genus (Pon/o- 

 poria) there may be as many as sixty of such teeth on 

 each side of each jaw, making 240 in all. The more 

 usual number is from twenty to thirty. These teeth are 

 never changed, being "monophyodont " and they are, 

 moreover, less firmly implanted in the jaws than in land 

 mammals, having never more than one root, which is set 

 in an alveolar socket which is generally wide and loosely 

 fitting, though perfectly sufficient for the simple purpose 

 which the teeth have to serve. 



Most singular modifications of this condition of denti- 

 tion are met with in different genera of toothed whales, 

 chiefly the result of suppression, sometimes of suppression 

 of the greater number, combined with excessive develop- 

 ment of a single pair. In one large group, the Ziphioids, 

 although minute rudimentary teeth are occasionally found 

 in young individuals, and sometimes throughout life, in 

 both jaws, in the adults the upper teeth are usually en- 

 tirely absent, and those of the lower jaw reduced to two, 

 which may be very large and projecting like tusks from 

 the mouth, as in Mesoplodon, or minute and entirely con- 

 cealed beneath the gums, as in Hyperoodon, — an animal 

 which is for all practical purposes toothless, yet in which 

 a pair of perfectly formed though buried teeth remain 

 throughout life, wonderful examples of the persistence of 

 rudimentary and to all appearance absolutely useless 

 organs. Among the Delpliinida similar cases are met with. 

 In the genus Grampus the teeth are entirely absent in the 

 upper, and few and early deciduous in the lower jaw. But 

 the Narwhal exceeds all other Cetaceans, perhaps all 

 other vertebrated animals, in the specialisation of its den- 

 tition. Besides some irregular rudimentary teeth found 

 in the young state, the entire dentition is reduced to a 

 single pair, which lie horizontally in the upper jaw, and 

 both of which in the female remain permanently con- 

 cealed within the bone, so that this sex is practically 

 toothless, while in the male the right tooth usually re- 

 mains similarly concealed and abortive, and the left is 

 immensely developed, attaining a length equal to more 

 than half that of the entire animal, projecting horizontally 

 from the head in the form of a cylindrical or slightly 



tapering pointed tusk, with the surface marked by spiral 

 grooves or ridges. 



The meaning and utility of some of these strange modi- 

 fications it is impossible, in the imperfect state of our 

 knowledge of the habits of the Cetacea, to explain, but 

 the fact that in almost every case a more full number of 

 rudimentary teeth is present in early stages of existence, 

 which either disappear, or remain as concealed and 

 functionless organs, points to the present condition in the 

 aberrant and specialised forms as being one derived from 

 the more generalised type, in which the teeth were 

 numerous and equal. 



The Mystacocetes, or Whalebone Whales, are distin- 

 guished by entire absence of teeth, at all events after 

 birth. But it is a remarkable fact, first demonstrated by 

 Geoffrey St. Hilairt, and since amply confirmed by 

 Cuvier, Eschricht, Julin, and others, that in the foetal 

 state they have numerous minute calcified teeth lying 

 in the dental groove of both upper and lower jaws. 

 These attain their fullest development about the middle 

 of fcetal life, after which period they are absorbed, no 

 trace of them remaining at the time of birth. Their 

 structure and mode of development has been shown to be 

 exactly that characteristic of ordinary mammalian teeth, 

 and it has also been observed that those at the posterior 

 part of the series are larger, and have a bilobed form of 

 crown, while those in front are simple and conical, a fact 

 of considerable interest in connection with speculations 

 as to the history of the group. 



It is not until after the disappearance of these teeth 

 that the baleen, or whalebone, makes its appearance. 

 This remarkable structure, though, as will be presently 

 shown, only a modification of a part existing in all mam- 

 mals, is, in its specially developed condition as baleen, 

 peculiar to one group of whales. It is therefore perfectly 

 in accord with what might have been expected, that it is 

 comparatively late in making its appearance. Characters 

 that are common to a large number of species appear 

 early, those that are special to a few, at a late period ; 

 alike both in the history of the race and of the indi- 

 vidual. 



Baleen consists of a series of flattened, horny plates, 

 several hundred in number, on each side of the palate, 

 separated by a bare interval along the middle line. They 

 are placed transversely to the long axis of the palate, 

 with very short spaces 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. The blades are 

 longer near the middle of the series, and gradually 

 diminish near the front and back of the mouth. The 

 horny plates grow from a dense fibrous and highly vas- 

 cular matrix, which covers the palatal surface of the 

 maxillae, and which sends out lamellar processes, one of 

 which penetrates the base of each blade. Moreover, the 

 free edge of these processes is covered with very iong 

 vascular thread-like papillae, one of which forms the 

 central axis of each of the hair-like epidermic fibres of 

 which the blade is mainly composed. A transverse 

 section of fresh whalebone shows that it is made up of 

 numbers of these soft vascular papillae, circular in outline, 

 each surrounded by concentrically arranged epidermic 

 cells, the whole bound together by other epidermic cell*, 

 which constitute the smooth cortical (so-called "enamel") 

 surface of the blade, and which, disintegrating at the free 

 edge, allows the individual fibres to become loose and to 

 assume the hair-like appearance spoken of before These 

 fibres differ from hairs in not being formed in depressed 

 follicles in the enderon, but rather resemble those of 

 which the horn of the rhinoceros is composed. The 

 blades are supported and bound together for a certain 



