ORDINAL TYPES OF MOLARS 115 



MOLAR TEETH OF MULTITUBERCULATES, MARSI.TIALS (?), AND PLACENTALS 

 ^1 IN THE UPPER CRETACEOTS. 



Some years ago the interesting discovery was made of a great number 

 and variety of teeth in fragmentary jaws of mammals in beds of Upper 

 Cretaceous age in North America. The specimens are so fragmentary 

 and isolated that it is difficult to classify them ; some probably represent 

 Placentals, others probably represent Marsupials, whilst still others are 

 possibly Prototheria (Multituberculata). 



As shown in Figs. 47 and 55, from Osborn, among these forms are two 

 prevailing types of teeth, namely, the highly specialized mxltitt/lerculate 

 teeth, which had reached almost their final extreme of evolution, and the 

 less specialized trituberculate teeth which are in a comparatively early 

 stage of evolution. 



The Multituberculates (see p. 95 and Fig. 55) prove that regularly 

 succi'xxi r<- cn*i> ii'Iilifion had taken place since the earlier Jurassic stage, as 

 a result of which the first upper and lower molars exhibit as many as 23 and 

 12 tubercles respectively in Ptilodus, or 21 cusps and 9 cusps respectively 

 in Meniscoessn*. This law of successive cusp addition from the posterior 

 basal cingulum is entirely analogous to that which occurs in the complicated 

 molars of the Proboscidea. It demonstrates that cusp origin in the Multi- 

 tuberculata at least is not by concrescence but by cusp addition. 



Trituberculates. In this group (see also pp. 95-97, Fig. 47), all the 

 upper molars known are of the simple, generally low- crowned, tritubercular 

 type, that is, they consist of the trigon to which no trace of the hypocone 

 or postero-internal tubercle has been added. About ten distinct kinds of 

 upper molars have been found altogether. They are all much more recent 

 in type than the few upper molars known of Jurassic mammals, which are 

 invariably high-crowned or piercing ; on the other hand, they are some- 

 what older in type than the prevailing molar teeth of the basal Eocene 

 or Puerco, because they lack all traces of the hypocone, which is very 

 common in Puerco teeth. These Cretaceous up^cr molars exhibit, how- 

 ever, two other characters found also in the Jurassic, namely, (1) small 

 comdi's or intermediate tubercles; (2) external sft/fcs, or extensions of the 

 cingulum on the outer wall ; in some cases these styles are very 

 prominent, forming distinct cusps (Fig. 47). 



The primitive features of the upper molars all point toward a tri- 

 tubercular ancestry. We observe first the large size of the internal 

 cusp or protocone, which certainly supports the theory that this cusp 

 was earlier in evolution than either of the outer cusps.* Second, we 

 observe the striking symmetry of the external cusps or paracone and 

 metacone, giving a symmetrical form to the trigon or main triangle 



*[See criticism of the similar view expressed on p. 95. ED.] 



