352 MAMMALIA [DENTAL Arrange- fication of teeth, The complete series of permanent teeth of most mammals forms a complex machine, with its several parts adapted for different functions, the most obvious structural modification for this purpose being an increased complexity of the individual components of the series from the anterior towards the posterior extremity of such series. Since, as has just been said, the complete series of the milk teeth j often presents structurally and functionally a similar machine, but composed of fewer individual members, and the anterior of which are as simple, and the posterior as complex, as those occupying corresponding positions in the permanent series, and since the milk teeth are only developed in relation to the anterior or lateral, never to the most posterior of the permanent series, it follows that ; the hinder milk teeth are usually more complex than the teeth of which they are the predecessors in the permanent series, and represent functionally, not their immediate suc cessors, but those more posterior permanent teeth which have no direct predecessors. This character is clearly seen in those animals in which the various members of the molar series are well differentiated from each other in form, as the Carnivora, and also in Man. In animals which have two sets of teeth the number of the teeth of the permanent series which are preceded by milk teeth varies greatly, being sometimes, as in Marsupials and some Rodents, as few as one on each side of each jaw, and sometimes including the larger portion of the series. Although there are difficulties in some cases in arriving at a satisfactory solution of the question, it is, on the whole, safest to assume that when only one set of teeth is present, these correspond to the permanent teeth of the Diphyodonts. When this one set is completely developed, and remains in use throughout the animal s life, there can be no question on this subject. When, on the other hand, the teeth are rudimentary and transient, as in the Whale bone Whales, it is possible to consider them as representing the milk series ; but there are weighty reasons in favour of the opposite conclusion. 1 General Arrangement, Homologies, and Notation of Teeth f Mammals. The teeth of the two sides of the jaws are a ^ wa y s alike in number and character, except in cases of accidental or abnormal variation, and in the one remark able instance of constant deviation from bilateral symmetry among mammals, the tusks of the Narwhal (see fig. 49, p. 398), in which the left is of immense size, and the right rudimentary. In those animals also, as the Dolphins and some Armadillos, which have a very large series of similar teeth, not always constant in number in different indi viduals, there may be differences in the two sides ; but, apart from these, in describing the dentition of any mammal, it is quite sufficient to give the number and characters of the teeth of one side only. As the teeth of the upper and the lower jaws work against each other in masticating, there is a general correspondence or harmony between them, the projections of one series, when the mouth is closed, fitting into corresponding depressions of the other. There is also a general resemblance in the number, characters, and mode of succession of both series, so that, although individual teeth of the upper and lower jaws may not be in any strict sense of the term homologous parts, there is a great convenience in applying the same descriptive terms to the one which are used for the other. The simplest dentition as a whole is that of many species 1 This and other questions concerning the homologies, notation, and succession of the teeth of mammals are more fully developed in two memoirs by the present writer : " Remarks on the Homologies and Notation of the Teeth of the Mammalia," in the Journal of Anatomy and Physiology, vol. iii. p. 262, 1869; and " Notes on the First or Milk Dentition of the Mammalia," in the Trans, Odontological Society of Great Britain, 1871. of Dolphin (fig, 2), in which the crowns are single-pointed, slightly curved cones, and the roots also single and taper ing, and all alike in form from the anterior to the posterior end of the series, though it may be with some slight difference in size, those at the two extremities of the FIG. 2. Upper and Lower Teeth of one side of the Mouth of a Dolphin (Lageno- rhynchus), as an example of the homodont type of dentition. The bone eover- ing the outer side of the roots of the teeth has been removed to show their simple character. series being rather smaller than the others. Such a den tition is called Homodont, and in the case cited, as the teeth are never changed, it is also Monophyodont. Such teeth are adapted only for catching slippery living prey, as fish. In a very large number of mammals the teeth of different parts of the series are more or less differentiated in character, and have different functions to perform. The front teeth are simple and one-rooted, and are adapted for cutting and seizing. They are called "incisors." The back teeth have broader and more complex crowns, tuberculated or ridged, and they are supported on two or more roots. They crush or grind the food, and are hence called "molars." Many animals have, between these two sets, a tooth at each corner of the mouth, longer and more pointed than the others, adapted for tearing or stabbing, or for fixing struggling prey. From the conspicuous development of such teeth in the Carnivora, especially the Dogs, they have received the name of " canines." A dentition with its component parts so differently formed that these distinctive terms are applicable to them is called Heterodont. In most cases, though by no means invariably, animals with Heterodont dentition are also Diphyodont. This general arrangement is extremely obvious in a considerable number of mammals ; arid closer examination shows that, under very great modifications in detail, there is a remarkable uniformity of essential characters in the dentition of a large number of members of the class belonging to different orders and not otherwise closely allied, so much that it has been possible (chiefly through the researches of Professor Owen) to formulate a common plan of dentition from which the others have been derived by the alteration of some and suppression of other members of the series, and occasionally, but very rarely, by addition. The records of palaeontology fully confirm this view, as by tracing back many groups now widely separated in dental characters we find a gradual approxi mation to a common type. In this generalized form of mammalian dentition (which is best exemplified in the genera Anoplotherium and HomalodontotJieriwn) the entire number of teeth present is 44, or 11 above and 11 below on each side. Those of each jaw are placed in continuous series without intervals between them; and, although the anterior teeth are simple and single-rooted, and the pos terior teeth complex and with several roots, the transition between the two kinds is gradual. In dividing and grouping such teeth for the purpose of description and comparison, more definite characters are required than those derived merely from form or function. The first step towards a classification has beeL. made by the observation that the upper jaw is composed of two bones, the premaxilla and the maxilla,
and that the suture between these bones separates the