OCR Text |
Show 1899.] PLEXOUONT MOLARS OF MAMMALS. 561 are limited to the internal margin. A somewhat similar disposition is seen in the molars of some Cretaceous Diprotodonts of the family Garzonidae, e. g. the molar of Halmariphus guaraniticus, represented in fig. 5. This agreement in the disposition of the primitive molar elements seems to imply that the Rodents, the origin of which is still a mystery, may represent a side branch of the Diprotodonts, which originated towards the middle of the Cretaceous period. W e may next consider the Ungulates, which by their molars, at least those of the present epoch, do not appear to bear any relation to the Didelphyidae aud their predecessors. This, however, is not the case. In a recent publication, I have declared that in the Cretaceous of Argentina all the groups of Ungulates exhibit in the form of their molars a great resemblance to each other: ail show the sexcuspidate form ; if not visible in the adult, it is seen iu young stages. The Argentine Proterotheridae, resembling the Horses in their tridactyle and even monodactyle hoofs, and the Palaeotheridae in their molars, are amongst the most characteristic and most specialized of Ungulata. Their oldest known representative is the Deuterotherium distichum of the Upper Cretaceous ; its fifth right lower molar, just in the beginning of wear, is represented from the upper aspect in fig 7 a. This tooth shows the six conical and Deuterotherium distichum: fifth right lower molar, twice nat. size.-Upper Cretaceous; Patagonia, a, Superior aspect of tooth which has just pierced the gum ; /;, superior aspect of worn tooth of adult; c, external aspect of slightly worn tooth. perfectly separated cusps, with a disposition closely similar to that of Proteodidelphys, and also with the cingulum (c) on the external side, visible in figure 7 c, which represents a slightly worn specimen of the same tooth. However, in the present genus this conformation had become transitory, as shown by the figure, lb, which exhibits the crown view of the same tooth in a worn condition; tbe positions formerly occupied by the primitive elements are marked by the corresponding letters, but the cusps are no longer recognizable, and without being acquainted with the unw'orn tooth it could not be guessed that its starting point is almost absolutely identical wdth the form presented by the same tooth of the Didelphyidae and of Proteodidelphys. The last-named |