116 EVOLUTION OF MAMMALIAN MOLAR TEETH 



of cusps,* which also lends strong support to the theory that the large 

 protocone was the original cusp of a trigon and that the para- and 

 metacones were added as lateral or external cusps of equal size. The 

 " premolar analogy " or " paracone theory," on the other hand, receives no 

 support whatever from the study of these superior molar teeth. 



In general it may be said that the upper molars are intermediate 

 between the Jurassic and the basal Eocene stages of evolution. They 

 thus lend overwhelming proof, if any more were needed, of (1) the 

 progressive evolution of trituberculy among the mammals, (2) of the law 

 of adaptation of the crown by cusp addition. These principles are equally 

 well illustrated in the lower molars. 



The lower molars (Fig. 47, Al, HI, etc.) are relatively more pro- 

 gressive, because (1) not only is the broad heel or talonid well developed, 

 but on this heel the three typical cusps, hypoconid, hypoconulid, and 

 entoconid, are in some cases present. (2) Another progressive character 

 is that the usually elevated trigonid has already been modified in one 

 type (Fig. 47, II) by the degeneration of the antero-internal cusp or 

 paraconid, although possibly the small size of the paraconid may be a 

 primitive character. (3) These lower teeth are of two types ; first, the 

 secodont or tuberculo-sectorial with elevated triangle or trigonid and a 

 lower heel or talonid, as illustrated in Fig. 47, HI, Al, El; second, 

 a slightly more bunodont type, in which the trigonid has become 

 secondarily depressed although still above the talonid. This secondary 

 depression is carried much further in certain Primates and Ungulates 

 of the basal Eocene. All the other lower molars present unusual 

 features. The one marked HI has an extremely high paraconid, a feature 

 noticeable, though in a less degree, in the two marked Al. On the other 

 hand in No. II. the paraconid is small and the metaconid is higher 

 than the protoconid.t In the upper molars, No. 7 may be regarded 

 either as a fourth premolar of the type seen in the Creodont, Pseudop- 

 terodon minutus, or as a molar of the type seen in the Oligocene 

 insectivore Ictops tJwmsoni (Fig. 66). A noticeable feature of all the 

 upper molars is the indentation in the middle of the outer border of 

 the crown, as in Peralcdes (Fig. 12), Kurtodon (Fig. 13), Dryolestes 

 (Fig. 14), and many modern trituberculate insectivores (Figs. 65-80). 



SPECIAL REFERENCES. 



Osborn, H. F., "Fossil Mammals of the Upper Cretaceous Beds," Bull. Amer. 

 Mm. Nat. Hist., Vol. V., 1893, pp. 311-330. 



*[A marked advance upon the condition seen in the Upper Jurassic Dryolestes, 

 where the paracone was large and medio-external instead of at the anterior tip of the 

 triangle. ED.] 



t [The metaconid is almost directly internal to the protoconid and connected with it 

 by a transverse cutting crest, as in Amblothermm and other Upper Jurassic genera. ED.] 



