TRITUBERCULY 57 



But this may be only a proof that the nmltitubercular ante- 

 dates the tritubereular. It may be, indeed, that the mammalian 

 tooth ^yas already differentiated among the mammal-like Sauriaiis, 

 and that from such a form as Oynngnnthus the Eutheria and other 

 forms in which a tritubercular arrangement can be detected were 

 evolved, and from such form as Trityloclon the Monotre- 

 matous branch of the mammals. This way of looking at the 

 matter harmonises a much -disputed question, l:iut involves 

 a diphyletic origin of the mammals — an origin which for other 

 reasons is not without its supporters. 



We shall now attempt to give a general idea of the facts and 

 arguments which support or tend to support " trituberculy." As 

 a matter of fact the name is inaccurate ; for the holders of this 

 view do not derive the mammalian molar from a trituberculate 

 condition, but in the first place from a simple cone such as that 

 of a crocodile ! 



To this main and at first only cusp came as a reinforcement 

 an additional cusp at each side, or rather at each end, having 

 regard to their position with reference to the long axis of the 

 jaw. This stage is the " triconodont ' stage, and teeth exist among 

 living as well as extinct mammals which show this early form of 

 tooth. We have, indeed, the genus Triconodon, so named on that 

 very account. Among living mammals the Seals and the Thyla- 

 cine all show some triconodont teeth. A Toothed Whale, it may 

 be remarked, is a living example of a mammal with monocono- 

 dont teeth. The three primary cusps, as the supporters of Co])e's 

 theory of trituberculisni denominate them, are termed respectively 

 the protocone, paracone, and metacone, or, if they are in the teeth of 

 the lower jaw, protoconid, paraconid, and metaconid. At a slightly 

 later stage, or coincidently, a rim partly surrounded the crown of 

 the tooth ; the rim is known as the cingirlum, and from a pro- 

 minent elevation of this rim a, fom-th cusp, the hypocone, was 

 developed. The three main cones then moved, or rather two of them 

 moved, so as to form a triangle ; thi^ is the tritubercular stage. 

 Teeth of this pattern are common, and occur in such ancient 

 forms as Insectivora and Lemurs, besides numerous extinct grou]^^)s. 

 An amendment has been suggested, and that is to term the teeth 

 with the simple primitive triangle " trigonodont," and to reser-\'e 

 the term tritubercular for those teeth in which the hypocone has 

 appeared. The platform bearing the hypocone widened into the 



