SKC(tXI) orTUXK OF TIIITrr.KIU UA 7'.* 



stages are " haplodont," " triconodout," " tritubercular," so the retrogressive 

 stages reverse this order, passing from "tritubercular" back to " tricono- 

 dont " then into " haplodont." Another view therefore is that such primary 

 forms have been secondarily acquired. The apparently " triconodont ' 

 lower molar of Thylaci/nux is, for example, an indirect retrogression from 

 a tritubercular ancestral form. Again among the aquatic carnivora, in 

 the series of molars of the Seals, the eared Seals and the Walruses, we see 

 the backward stages from the " triconodont " to the " haplodont " ; and it 

 is therefore probable that the " tritubercular " was the form of molar 

 possessed by the Pinnipedia when they diverged from the Fissipedia. 

 There is considerable evidence that a similar retrogression has simplified 

 the molar crowns of modern Edentates, for it is now certain that at least 

 the Gravigrada were descended from tritubercular ancestors, the Gano- 

 donta [Ta?niodonta], Again, among the Cetacea, all their oldest allies, 

 such as Zeuglodon, are triconodont, not haplodont. With both these 

 groups, therefore, there are the possibilities of direct or of retrogressive 

 origin of the " triconodont " molar. 



This uncertainty hardly extends to the " triconodont " stage, which is 

 typically shown in the Lower Jurassic Ampliilestcs, Phascolof/n r//<m and 

 the later Trin<,i<!n,i. It is a very significant fact that this type dies out 

 in the Upper Jurassic. It is true we find many more recent " tri- 

 conodont " teeth, the lower molar of Mcsonyx for example, which are 

 positively known to be of tritubercular origin. Richard Owen compared 

 the lower molars of Thylaciims with those of Triconodon, but we have 

 found that what appeared to him to be similar cusps are not really 

 homologous. Thus while it is possible that the ancestors of some of the 

 modern haplodent and triconodont mammals never reached the tri- 

 tubercular stage, it is by no means a settled fact. On the other hand, 

 excepting the isolated group of Multituberculates and the single genus 

 Dic/'iirt/iiiiifn/i Marsh, the- molars of crcri/ l-nou'n foxsil mamma/* from /In- 

 close of the Ln/'-rr Cretaceous until tin- clow f Ilir E<H'<'H<- [>< '//<></ bear ///< 

 trituh rcular *fn>/>. 



This would appear to support the generalization that all mammals 

 passed through the third primary or tritubercular stage, yet it must 

 be borne in mind that all our evidence is derived from inhabitants 

 of fresh water basins,t and that the persistent haplodont and triconodont 

 types may have been living contemporaneously in the seas. 



But the Multituberculates and Monotremes, were they tritubercular 

 in origin ? The teeth of Ornithorhynchus are so degenerate and irregular 



* [This statement applies chiefly to the orders Insectivora, Carnivora, Primates, 

 Ungulata, since the trituberculate derivation of the Monotremata, Multituberculata, 

 Edentata, Rodentia, Cetacea, remains to be proven. ED.] 



t [Or at least from epicontinental as opposed to marine deposits. ED.] 



