The homologous teeth being thus deter minable, they may be severally signified by a symbol as well as by a name. The incisors, e.g., by their initial letter i., and indivi dually by an added number, i. 1, i. 2, and i. 3; the canines by the letter c.; the pre molars by the letter p.; and the molars by the letter na.; these also being differentiated by added numerals. Thus, the number of these teeth, on each side of both jaws, in any given species, Man e. g., may be expressed — by the following brief formula : — 2 2 22 ' 1— 2-2 3-3 C. p. - M. - =Z; and the 11011110• 2-2 3-3 looies of the individual teeth, in relation to the typical formula, may be signified by i. 1., i. 2.; c. ; p. 3., p. 4.; m. 1., an. 2., m. 3.: the suppressed teeth being i. 3.*, p. 1., and p. 2.
Examples of the typical dentition are ex ceptions in the actual creation ; but it was the rule in the forms of Mammalia first introduced into this planet ; and that, too, whether the teeth were modified for animal or vegetable food. Fig. 576., e g., shows the dental series The true molars in the one are tuberculate, indicating its tendency to vegetable diet; in the other, they are carnassial, and betoken a peculiarly destructive and bloodthirsty spe cies.
In the Quarterly Geological Journal, No. 13, 1848, p. 36. pl. iv., I have described and figured the entire dental series of one side of the lower jaw of an extinct hoofed quadruped, the Dichodon cu.pidatus, from eocene or oldest tertiary strata, also mani festing the normal number and kinds of teeth, but with such equality of height of crown, that no interspace is needed to lodge any of the teeth when the jaws are closed, and the series is as entire and uninterrupted as in the human subject. A great proportion of the upper jaw and teeth has been discovered, and of the upper jaw of the Amphieyon major, a mixed-feeding ferine animal, allied to the Bear. Fig. 577. shows the dental series of the under jaw of a more strictly carnivorous the marks of abrasion on the lower teeth prove the series above to have been as entire and continuous as that below. The Anoplo therium (" Odontography," pl. 135. fig. 1.), beast, the ilyernodon ; the fossil remains of a species of which have been discovered in the oldest tertiary deposits of Hampshire. The symbols denote the homologies of the teeth.
from the gypsum quarries of Montmartre, geologically as ancient as the eocene clays of this island, long ago presented to Cuvier the same peculiar continuous dental series as is shown in the Diehodon. In his original Me moir, Cuvier described the canines as a fourth pair of incisors, on account of their small size and their trenchant shape; but he after wards recognised their true homology with the larger and more laniariform canines of the Pakeotheriuni (" Odontography," pl. 135., 4.). The Choropotamus*, the ilnthraco theriunzt, the Hyopotantus I, the Hyracothe rium§, the Oplotheriunt, the Merycopotanzus, the Hippohyus, and other ancient (eocene and miocene) tertiary mammalian genera presented the forty-four teeth, in number and kind according to that which is here pro pounded as the typical or normal dentition of the placental Mammalia. Amongst the
existing genera, the hog (Sus) is one of the few that retain this type. Fig. 578. shows the entire permanent series, exposed, in both molar, in. 2, has just begun to cut the gum p. 2, p. 3, and p. 4, together with nt. 3, are more or less incomplete and concealed in their closed alveoli.
The premolars must displace deciduous molars in order to rise into place ; the molars have no such relations ; it will be observed, that the last deciduous molar, d. 4, has the same relative superiority of size to d. 3 and d. 2 which tn. 3 bears to nt. 2 and at. 1 ; and the crowns of p. 3 and p. 4 are of a more simple form than those of the milk teeth which they are destined to succeed.
Teeth of each of the kinds above deter jaws, and indicated individually by their symbols. Fig. 579. illustrates the phenomena of development which distinguish the pre mined, and arbitrarily named " incisors," nines," "premolars," "molars," have received other special names in regard to certain pe molars from the molars. The first premolar, p. 1, and the first molar, nt. 1, are in place and use, together with the three deciduous molars, d. 2, d. 3, and d. 4 ; the second culiarities of form or other property ; and the ablest comparative anatomists have been led astray in determining their homologies when they have suffered themselves to be guided exclusively by morphological characters. The premolars in the human subject have been called "bicuspids" The last upper pre molar and the first lower true molar in the Carnivora are termed, from their peculiar form, " sectorials," or " carnassial teeth," " molaires carnassieres" of Cuvier, Teeth of an elongated conical form, pro jecting considerably beyond the rest, and of uninterrupted growth, are called "tusks ; " such are the incisors of the Elephant and Dugong, the canines of the Boar and Walrus : the long and large incisors of the Rodents have been termed, from the shape and structure of their cutting edge, scalpriforrn or chisel-teeth, " denies sealprarii." The inferior in cisors of the flying Lemurs (Galeo inthecus) have the crown deeply notched like a comb, and are termed " denies pectinati." The canines of the Baboons are deeply grooved in front, like the poison-fangs, " denies canaliculati," of some serpents. The compressed conical crowns of the molar teeth of the small clawed seals (Stenorhynchus) are divided either like a trident, into three sharp points, or like a saw, into four or five points ; the molars of the great extinct Zeuglodon had a similar form ; such teeth have been called dente: serrati. But the philosophical course of the knowledge of nature tends to explode needless terms of art, invented for unimportant vari eties, and to establish and fix the meaning of those terms that are the signs of determinate species of things.