18 ME GAL ONYX. 



the angles strongly rounded. The anterior side is transverse and slightly convex, 

 and the inner side is also convex. The posterior side is slightly concave and 

 directed obliquely outward. 



The last molar is a smaller tooth than the preceding pair, but has nearly the 

 same form in a reversed position, the base of the triangular section (PI. XVI, Fig. 

 9, a), being outward. Its posterior side is transverse and is slightly concave ; and 

 the anterior side is convex and directed obliquely inward. 



The triturating surface of the last molar is broken off, but in the three preceding 

 teeth it presents the form of a transverse concave valley sloping inwardly. 

 Anteriorly and posteriorly the valley is bounded by an angular ridge formed from 

 the harder dentinal layer of the teeth with the cementum worn from it in a bevelled 

 manner. (PL III.) 



The posterior four superior molars in the skull of Dr. Dickeson's collection, are 

 very nearly like those juSt described. (PI. VI, Tig. 1; XVI, Fig. 10, d, c, h, a.) 

 The third molar is not as thick internally, and is a little thicker externally, so that 

 the antero-posterior diameter is more uniform. The fourth molar is less broad and 

 is more convex anteriorly ; and the last of the series is a little broader, and in sec- 

 tion is more elliptical. The triturating surface, which is preserved entire in the 

 specimen of the latter tooth, presents a transverse valley like that of the teeth in 

 advance. 



Measurements of tJie Posterior four Superior Molars in the Shull of Dr. Owen's 



Collection. 



TRANSVERSE. ANTERO-POSTERIOR. 



INT. EXT. 



, Lines. Lines. Lines. 



Diameter of second upper molar 9 8^ 7 



Diameter of third upper molar 12 9^ 6 



Diameter of fourth upper molar ...... 12|- 9 5 



Diameter of fifth upper molar 10 3 6i 



Measurements of the correspondi/ng Teeth in the Shidl of Dr. Dichesons Collection. 



TRANSVERSE. ANTERO-POSTERIOR. 



INT. EXT. 



Lines. Lines. Lines. 



Diameter of second upper molar 9^ . 8 1 



Diameter of third upper molar ...... 12^ 8 6 



Diameter of fourth upper molar ...... 11|- 8 6 



Diameter of fifth upper molar . ' . . . . . 10^ 4 6 



The isolated superior back molar (PI. XVI, Fig. 11), from Adams County, Mis- 

 sissippi, in the collection of Col. Wailes, approaches most in its form the fourth 

 upper molar contained in the skulls above described. Its transverse diameter is 

 eleven lines ; and its antero-posterior diameter externally six lines, and internally 

 seven and a quarter lines. 



The supposed isolated superior third molar (PL VI, Figs. 8, 9; XVI, Fig. 13), 

 from Tuscumbia County, Alabama, in the collection of Professor Wyman, is rela- 



