126 Royal Society. 



by an unusually early abrasion and expulsion, when its place is ob- 

 literated by the second true molar being pushed forwards into con- 

 tact with the last premolar. This tooth is as remarkable for its 

 longevity, and remains after the wearing away and shedding of the 

 second true molar, when the last true molar advances into contact 

 with the last premolar, and the place of both the previously inter- 

 vening true molars is obliterated. This unusual order of shedding 

 of the molar teeth has given rise to the idea of the last large and 

 complex true molar of the Phacochmrus being the homologue of 

 both the last and penultimate grinders of the common Hog, which 

 the author's observations refute ; and he, also, is able to point out, 

 by re-examination of the original specimen figured by Home in the 

 Phil. Trans., the source of the erroneous idea that the common Hog 

 had an additional true molar behind the large one symbolised by 

 m 3, in the author's system of dental notation. 



The nature and signification of the symbols proposed are ex- 

 plained and illustrated by a series of drawings. One of the fruits 

 of the determination of the homology of a part is the power of gi- 

 ving it a name, and signifying it by a symbol applicable co-exten- 

 sively with such homology. The limits are shown within which 

 the homologies of individual teeth can be determined : they present 

 the requisite constancy of character in a large proportion of the 

 class Mammalia. Certain members of this class, e.g, the order 

 Bruta and the Cetacea vera, have teeth too numerous and alike in 

 form and mode of development to admit of being determined indi- 

 vidually from species to species. Such mammalia have but one set 

 of teeth, and the author proposes to call them *Monophyodonts.' 

 On the other hand, the orders Marsupialia, Insectivora, Rodentia, 

 Ruminantia, Pachydermata, Carnivora, Cheiroptera, Quadrumana 

 and Bimana have two sets of teeth, and might be called collectively, 

 * Diphyodonts.' Of the permanent teeth of this division of mam- 

 malia» some succeed the deciduous teeth vertically, others come into 

 place behind one another in horizontal succession. The * incisors' 

 are determined by a character of relative position to the jaws and 

 to each other : so likewise the ' canines.' The remaining teeth are 

 divided into those which are developed in vertical relation to the 

 deciduous molars, and push them out, and those that have not such 

 relation, but follow each other horizontally : the term ' molar ' is re- 

 stricted by the author to these latter teeth, and that of ' premolar ' 

 to the former ones, which are always anterior to the molars. There 

 is a remarkable degree of constancy in the number of these different 

 kinds of teeth ; in the placental Diphyodonts, e. g. the ' incisors * 



3—3 

 never exceed - _^, i, e. 3 on each side of both jaws, the * ca- 



1 — 1 4—4 3—3 



nines ' fZrT» ^^^ premolars . __. , the molars Q~ij =44 ; and this 



the author regards as the typical formula of dentition in the great 

 proportion of the mammalian class above defined. It was rarely 

 departed from by the primaeval species that have become extinct, 

 and is modified chiefly by defect or loss of certain teeth in the ex- 



