16 ME GAL ONYX. 



The pulp cavity extends in an infundibular form about two inches from its 

 margin. 



The antero-posterior diameter of this tooth is seventeen and a half lines ; its 

 greatest transverse diameter, nine lines. 



The corresponding tooth preserved in the left side of the skull in Dr. Dickeson's 

 collection, possesses the same form, but differs in being strongly impressed by three 

 longitudinal lines upon its outer surface (Pis. IV, XVI, Fig. 2). Eemovedfrom its 

 socket, this tooth is observed to have been in its perfect condition not more than 

 two-thirds the length of those contained in the skull of Dr. Owen's collection, 

 probably because it belonged to an old animal, and was worn away without an 

 equal amount of reparation. This is rendered more likely from the fact that the 

 pulp cavity reaches within a half an inch of the triturating surface of the tooth. 



The antero-posterior diameter of the specimen is eighteen and a quarter lines.; 

 its greatest transverse diameter nine and three quarter lines. 



The specimen of a first upper right molar tooth (PI. VI, Figs. 4, 5 ; XVI, Fig. 4), 

 from the neighborhood of Memphis, Tennessee, in the collection of Prof. Silliman, 

 is more robust than those above described. Its outer side is also more plane and 

 presents only the faintest indication of longitudinal depressed lines. Its antero- 

 posterior diameter is nineteen lines ; its greatest transverse diameter nine and a 

 half lines. 



The specimen of a first upper right molar tooth (PI. XVI, Fig. 5), from Adams 

 Co., Mississippi, in the collection of Prof. Wailes, is more convex on its outer side 

 than in any of those previously mentioned, and it presents no trace of longitudinal 

 impressed lines. Its antero-posterior diameter is eighteen and a half lines ; its 

 greatest transverse diameter nine and three-quarter lines. 



The specimen of a first upper left molar tooth (PI. XVI, Fig. 6), from Natchez 

 Bluffs, Mississippi, in the collection of Prof. Wyman, is smaller than any of those 

 described ; but it has about the same form and proportions as that of Prof Wailes' 

 collection. Its antero-posterior diameter is sixteen lines ; its greatest transverse 

 diameter eight and three-quarter lines. 



The isolated first molar (PL VI, Figs. 6, 7; XVI, Fig. 3), found in company 

 with a fragment of a rib and an ungual phalanx, near Natchez, Mississippi, and 

 now contained in Dr. Dickeson's collection, before having had the opportunity of 

 inspecting any number of first molars of different individuals oi Megalonyx Jeffej-sonii, 

 I suspected from its differing so much from those examined, that it indicated a 

 different species, for which I proposed the name of Megalonyx potens} The tooth 

 is relatively narrower in comparison with its antero-posterior diameter, than in any 

 of those described, and its outer surface is more plane. The triturating surface is 

 worn off obliquely and slopes postero-internally. The antero-posterior diameter of 

 the specimen is twenty-one lines ; its greatest transverse diameter nine and a half 

 lines. 



The first molar of the Megalonyx, described by Dr. Harlan, appears to be that of 

 the upper left side.^ Its form corresponds with those in the skull of Dr. Owen's 



' Proc. Acad. Nat. Sc, VI, 1852, 117. " See page 4. 



