OBJECTIONS AND DIFFICULTIES AND OTHER THEORIES I'll) 



(2) The molar crowns are broadly transverse or triangular, and upon 

 the internal side of each is a large, conical, pointed cusp, j>r, supported by 

 a large stout fang, Fig. 206 A, m-6, m7 ; around the inner side of each 

 of these cusps is a delicate cingulum, Fig. 206 A, c. 



(o) The I'.i-ti'i'iinl portion of the broadly triangular crown is supported 

 on two smaller fangs, Fig. 206 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 ?me 

 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, c, 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. 



This palseontological evidence appears to lend no support to the 

 evidence of embryogeny that the paracone (_>) or antero-external cusp 

 is the oldest cusp. On the contrary, it appears to prove that the 

 pointed, conical, internally placed cusp supported upon a single stout 

 fang is the main cusp or protocone, serially homologous with the main 

 cusp of the simple preceding premolar teeth. 



Gidley s Rest it dy (1906) of Jurassic Mammals Supports Embryogeny 

 and the Premolar Analogy Theory. 



It will be seen that embryogeny and premolar analogy concur in 

 evidence that the reptilian cone is represented in the antero-external cusps 

 of the superior molars. Mr. J. W. Gidley * now brings palaeontology to 

 the support of this position. 



His conclusions are based on a fresh study of unworn superior 

 molars of Jurassic mammals in the U.S. National Museum, which were 

 not accessible to Osborn. These teeth are represented in Fig. 207, 

 which may be compared with the worn teeth of Fig. 206. 



1 Gidley, J. W., "Evidence bearing on Tooth-Cusp Development," Proc. Washington 

 Acad. ScL, Vol. VIII., 1906, p. 106. 



FIGURE 207. FROM GIDLEV. 



Xos. 1 and 1. Tricnnotlon? bimlcus Marsh (Atlantosaurus beds), left upper molars, m- and m 3 ; 

 crown and external views. Six times natural size (Xo. -ii'.'s, I'.S.N.M.). 



2, ~2/i, and 26. Dryohstes sp. (Atlantosaurus beds), left upper molars ; crown, external, and 



posterior views. Seven times natural size (No. 2s4">, U.S.X.M.). 



3. Druolc&t'S, first right upper molar, in' ; crown view. Eight times natural size (Xu. >::'.', 



U.S.X.M.). 



4 and 4a. Dicrocynodon sp. (Atlantosaurus beds), left upper molars ; crown and external 



views. Six times natural size (Xo. 2715, U.S.X.M.). 

 5, 5a, 56, and 5c. Paurodon sp. (Atlantosaurus beds), right lower molar, m._. ; rr,i\vn, external, 



internal, and posterior views. Kight times natural size (Xo. 273M, L'.S.X.M.). 

 0, (3, 06, and Gc. ? Pediomi/s sp. (Laramie beds), left upper molar ; ero\vn, external, posterior, 



and anterior views. Bight times natural size (Xo. 5Uti2, U.S.X.M.). 



7, 7a, and 76. Gen. et. sp. indt. (Laramie beds), left upper molar ; crown, external, and 

 anterior views. Eight times natural size (Xo. 507G, U.S.X.M.). 



