TRITUBERCULAR THEORY. 267 



definite individual entities idnit iti;dli- :md traceable in tin- 

 teeth of all the different niemln-rs of that series. The latest 

 results of Palaeontology suggest that this tracing of the homo- 

 logous cusps may even be extended from the teeth of one great 

 division to those of another; that, indeed, there are certain 

 fundamental cusps common to the complicated molars of all the 

 Eutheria. Some palaeontologists have not hesitated even to 

 identify the primitive ancestral " reptilian cone " throughout 

 the Eutherian orders, and to propose a definite nomenclature 

 for this and the secondary cusps by which it is accompanied. 

 They have propounded the Tritubercular Thefl*jE-acusjt 

 development, which reduces most of the known facts at leas^ 

 provisionally to order. 



This theory or doctrine (elaborated especially by the American 

 palaeontologists, Cope, Osborn, and Scott) will be readily under- 

 stood from the accompanying diagram (fig. 157), which repre- 

 sents ten successive plans of cusp-arrangement, those of the 

 upper jaw being indicated by round black dots, those of the 

 opposing lower jaw by circles. Fig. A represents four upper 

 teeth and three lower teeth as simple cones, the normal reptilian 

 condition ; in B three teeth of both jaws have each become 

 complicated by the addition of a pair of small lateral cusps, an 

 arrangement observed in Dromatherium and other Triassic jaws. 

 Fig. C shows the cusps well-separated to form a true triconodont 

 tooth (as in the Lower Jurassic Phascolotherium (fig. 150) and 

 Amphilestes) ; while D exhibits the lower lateral cusps thrust 

 inwards and the corresponding upper cusps outwards, to form 

 the tritubercular tooth (as in Spalacotherium). A talon is next 

 added to the tritubercular lower molar (E), as in the Lower 

 Jurassic Amphitherium ; and this gradually increases in relative 

 proportions (F, G), eventually bearing small new tubercles or 

 cusps. The talon crushes into the valley of the upper tooth, 

 so that each pair of opposed molars act mechanically as two 

 shears and one crusher an arrangement prevailing among most 

 lemurs, insectivores, and carnivores. The upper molars now 

 begin to develop a small talon and intermediate tubercles 

 (H) ; and eventually (/, J) all the cusps become comparatively 

 small and rounded tubercles on a crown, which soon loses its 



