FOSSIL MAMMALIA. 83 



sub-genus the antero-posterior extent of the four alveoli of the lower jaw nearly 

 equals four inches, and is relatively greater than in the Mylodon, although the 

 teeth are placed closer together; this is owing to their greater relative size. The 

 first molar tooth presents the simplest form ; its transverse section is a compressed 

 inequilateral triangle with the angles rounded off; the longest diameter of this 

 section which is parallel with the inner alveolar border is eleven lines, the trans- 

 verse diameter almost six lines ; the base or broadest side of the triangle is turned 

 inwards, and is slightly concave ; the two smaller sides are also slightly concave. 

 The second molar is placed more obliquely in the jaw ; the long axis of its 

 transverse section intersects at an acute angle that of the jaw itself; the trans- 

 verse section presents a compressed or oblong form, with the larger end next the 

 outer side, and the smaller end next the inner side of the jaw ; this end is simply 

 rounded, but the outer end presents a sinuosity, corresponding to a broad groove 

 which traverses the whole length of the outer side of the tooth ; the anterior, 

 which corresponds to the internal side or base of the transverse section of the 

 preceding molar, is slightly concave. 



The third molar has nearly the same form and relative position as the pre- 

 ceding; the long diameter of the transverse section is, in both, ten lines and a half; 

 the principal transverse diameter is, in the second molar five lines, in the third 

 nearly six ; the difference of form observable in these as compared with the two 

 middle grinders of the Mylodon is well marked ; in the latter these teeth are im- 

 pressed with a longitudinal groove on their inner sides ; in the Scelidothere they 

 have a similar impression along their outer but not along the inner side. 



In the last molar the resemblance is much closer, and the modification of 

 form by which it differs from the preceding ones is of the same kind ; the trans- 

 verse section gives an irregular oblong figure with its axis nearly parallel with 

 that of the jaw, and constricted at the middle by sinuosities produced by two wide 

 channels which traverse longitudinally, one the outer, the other the inner side of 

 the tooth ; the latter groove is much wider and shallower in the Scelidothere than 

 in the Mylodon. The two lobes produced by these grooves are more equal in 

 Scelidothere; the anterior one is concave on its anterior surface instead of convex 

 as in the Mylodon; the posterior one is more compressed; the longitudinal or 

 antero-posterior diameter of the transverse section of this tooth is one inch 

 five lines ; the greatest transverse diameter is nine lines ; the diameter of the 

 isthmus joining the lobes is three lines and a half; the entire length of this tooth 

 is three inches three lines.* 



* It requires little stretch of imagination to conceive that this more complex posterior tooth (PI. XXIII, 

 fig. 4, 4) in the lower jaw is the representative of the two smaller posterior teeth (ib. fig. 3, 4, and 5) of the 

 upper jaw conjoined. 



M 2 



