322 H. F. Q shorn — Original Tritubercular Theory. 



todon, fig. 3, in the British Museum in which the main cone is 

 internal, (4) a reference by Professor Marsh to the fact that 

 in the upper molars of Dryolestes the main cone is internal. 

 In each case the main cone was believed to be the protocone. 



The teeth last mentioned (4) were not personally examined 

 at the time, but through the kindness of the late Erofessor 

 Charles E. Beecher I was recently enabled to study them in 

 the Yale University Museum. 



The specimens consist of two superior series. They both 

 show that the large, single, main cusp of the crown is internal, 

 and in my opinion they present conclusive evidence of the 

 truth of the tritubercular theory as originally proposed. 



More in detail, the two specimens taken together show per- 

 fectly the structure of both the crowns. and fangs of seven 

 superior molar teeth, and confirm entirely the general descrip- 

 tion given by Marsh,' to which some important points may be 

 added as follows : 



(1) The molars are sharply distinguished from the pre- 

 molars, which are 'bifanged teeth with simple, laterally coin- 

 pressed crowns. (2) The molar crowns are broadly transverse 

 or triangular, and upon the internal side of each is a large, 

 conical, pointed cusj}, pr, supported by a large, stout fang, fig. 

 1 A, m6, m7; around the inner side of each of these cusps is a 

 delicate cingulum, fig. 1 A, c. (3) The external portion of the 

 broadly triangular crown is supported on two smaller fangs, 

 fig. 1 A, m6, m7. (4) The external portion of the crown is 

 depressed, and bears one large antero-external cusp ?pa and 

 one smaller postero-external cusp fine which is either partly 

 worn away or less pronounced in development. (5) Outside 

 of this external wall there is also a faint basal cingulum, <?, <?, c. 

 (6) Connecting these low external cusps with the elevated 

 internal cusp are two transverse ridges; the anterior transverse 

 ridge is higher and stronger than the posterior. 



These features are clearly shown in the accompanying draw- 

 ings, fig. 1, which were made and shaded under the camera 

 lucida and therefore admit of no doubt as to interpretation. 



These two specimens fully supplement and confirm each 

 other ; they also supplement the evidence derived from the 

 study of the superior molar teeth of Peralestes (fig. 2) and of 

 Kurtodon (fig. 3) in the British Museum, which were cited and 

 figured in my memoir and in various subsequent papers on tri- 

 tuberculy. 



Again summing up this combined evidence, we find in the 

 Jurassic period the superior molars of the only mammals 

 known (excepting the Triconodonta and Multitnberculata) to 

 consist of a large conical internal cusp or protocone ', which Ave 



