GEOLOGICAL SUCCESSION OF MOLAR TYPES 97 



primitive feature is the entire absence of an internal cingulum and con- 

 sequent absence of any cusp corresponding with the hypocone ; in fact, 

 all these teeth are still strictly triangular in form, although the outer wall 

 may be somewhat irregular. A prominent cutting metastyle, analogous 

 to that seen in primitive Creodonts, is developed. 



In the lower molars we observe an elevated anterior triangle or 

 trigonid. As a rule the protoconid or reptilian cone is the most 

 prominent cusp, but in certain specimens (Fig. 47 HI} this cusp is some- 

 what depressed. A progressive character is the reduction of the antero- 

 internal cusp, or paraconid. But the most strikingly progressive feature, 

 as compared with Jurassic molars, is the broadening out of the heel of the 

 crown or talonid to support three cusps, the hypoconid, hypoconulid, and 

 entoconid respectively.* 



Thus there are three main features, as compared with the Jurassic 

 molars : (1) general depression of the crown of the upper molars, 

 accompanied by a strong development of the external cint/t/lar cvxj>* 

 and of conules, but not by an internal cingular development ; therefore 

 there is (2) an entire absence of the postero-internal cusp or hypocone ; (3) 

 there is a striking development of the heel or talonid of the lower molars 

 with two or three cusps, and more or less depression of the trigonid. In 

 general, these progressive features relate these teeth very much more 

 closely to those of the Basal Eocene than to those of the Upper Jurassic. 



Numerically, the most progressive of these upper molars have from 

 two to three fangs ; from three to five cusps, namely protocone, paracone, 

 metacone, protoconule and metaconule ; and two to three cingules, 

 parastyle, mesostyle, and metastyle ; the lower molars have two or more 

 fangs and from five to six cusps, protoconid, paraconid, metaconid, 

 hypoconid, hypoconulid and entoconid. 



* [This broadening of the talonid was probably con-elated with the depression and 

 transverse broadening of the protocone. ED.] 



In Figs. A, B, D, the long sectorial spur from the outer wall is more probably the metastyle 

 than the parastyle, consequently this is probably the posterior side of the tooth ; although if the 

 tooth happened to be in'>, the elongate spur would be anterior, ;., . parastyle. 



A, Crown and side views of three superior molars, probably of the right side. Types of 

 Pi'OtolnmW.a liatclu.ri Osborn. 



B. Crown views of four superior molars of the left and right sides. Genus / ) ?<'n/////.< Marsh. 



C. A superior molar, and an inferior molar of the right side. Type of Sijiifunmloii sexicuspi 

 ( isliorn. 



D. Crown and anterior views of two superior molars of the left side. Genus not determined. 



E. Crown view of a right superior molar. bi<l< l/i/m/ix vorax Marsh, after Marsh. 



F. Crown views of two superior molars, probably of the left side. Type of Ertoconn,i:>,i 

 t i> t: rsoni Osborn. 



O. Crown and side views of a superior premolar or molar (ef. Jiri/nlixtis, Iciojix). Genus 

 not determined. 



El. An inferior molar of the left side. ? Di<i, Iphodon. Crown and inside views. 

 A!. Two inferior molars of the left side. Genus undetermined. Crown and inside views. 

 HI. Crown and inside views of a lower molar of the left side. Genus not determined. 

 II. Crown and inside views of a lower molar of the left side. Genus not determined. 



G 



