92 DR. B. A. BEXSLEY ON TEE EVOLUTION 



Sequence of Molar Patterns. — The molar cliaracters are remarkably constant in 

 tlie smaller species of the Dasyuringe ; so that, so far as these teeth are concerned, 

 any one of them might he selected as representing the starting-point for the insectivorous- 

 cavnivorons evolution. The following description is based on Smiiithopsis leueopm, the 

 teeth of which are represented in Pis. 5 & 6 by figs. 2 «, 6 of the second upper and third 

 loAver molars respectively. 



The second upper molar presents the following cliaracters: — The biting-surface is 

 triangular in outline, with the apex of the triangle directed internally. The latter is 

 separated from those of the neigliboui-ing teeth in front and behind by triangular 

 spaces, into which are inserted, wiien the jaws are closed, the anterior triangular 

 l)illars of the loAver teeth. The longest axis of the crown is transverse, so that the 

 tooth appears to be slightly compressed in an antero-posterior direction. The crown 

 presents the trituberculate * pattern in general common to the insectivorous Dasyuridse 

 and Didclphyida;, and also to the placental 'J'alpidai, the primitive Creodonta, and 

 the Cretaceous Mammalia. It bears three main cusps and an outer row of subsidiary 

 styles. Of the three main cusps, that placed internally, the protocone {pr., PI. 5. 

 fig. 2a), is well developed and is supported on a separate root; its crown surface is 

 placed slightly below that of the two remaining cusps of the triangle. Of tlie latter 

 the anteriorly situate paracone {pa.) is comparatively small, while the posteriorly placed 

 metacone {me.) is greatly enlarged and forms the most important element of the crown. 

 Its posterior border is modified so as to form a trenchant spur, which shears against the 

 antero-external border of the triangular pillar of the succeeding lower tooth. The 

 shearing-edge is relatively short, and its direction is for the most part transverse. All 

 three cusps are of a piercing or insectivorous type, their tips being triangular in section 

 and sharply jiointed. 



The structures referred to as external styles are practically two in number, and are 

 separated by a considerable spaci from the outermost of the main cusps. One of 

 them {ab), placed opposite the paracone, shows indications of a composite structure. 

 The remaining style {c) is much better developed ; its tip projects almost to the same 

 extent as that of the adjacent metacone. 



* Objection to this designation has been taken by Dr. Forsyth Major (1S93, p. 199) on the grounds that so-called 

 trituberculate teeth are often in reality polybunous. The justice of this view is obvious, such a tooth as that of 

 Hininihopsis hucopits being no more actually trituberculate than (if one may be allowed to cite an analogous case 

 of descriptive inapplicability) the Edentata are edentulous. The term is, however, of too great descriptive 

 convenience to be easily dispensed with when employed to designate such teeth as those presenting three main cusps 

 arranged in the form of a triangle, with or without a complement of intermediate conules or external styles. 



Descriptive terms such as the present indicate definite states rather than progressions, and the evolutionist 

 is hampered by the necessity either of extending their meaning to more than they literally signify, or of adopting a 

 cumbrous nomenclature to designate minor modifications. As to how far a tooth may be modified away from its 

 original type and still be designated as trituberculate is a question of individual opinion. The term can obviously 

 not be applied to those teeth which have passed the tuberculate stage in the carnivorous evolution, and should not 

 be applied to those which possess an incipient hypocone (r/. PI. 5. fig. S), indicative of an omnivorous evolution, 

 in view of the fact that the latter element, in subsequent stages, comes to rank in importance with the three 

 original cusps. 



