262 ON PLATYGONUS COMPRESSUS : 



All these teeth, by wearing, lose the separation between the 

 cusps of the transverse elevations, which thus become broad and 

 straight ridges, haVing the extremities a little more elevated than 

 the middle. 



In the fragment of the lower maxilla, only the second and third 

 molars are preserved. There are remains of the first molar and the 

 posterior premolar, but not sufficient for description. 



The second molar, m. 2 (fig. 7), is quadrangular, with rounded 

 extremities and somewhat emarginate sides ; it presents two large 

 transverse ridges separated bj a deep valley ; there is a very slight 

 anterior and posterior basal margin, more elevated in the middle ; the 

 valley has a very indistinct margin externally, and at that place the 

 anterior lobe rises suddenly, so as to form a very well defined right 

 angle with the margin ; there is another angle, but less sharply 

 defined, between the same margin and the posterior lobe. 



The third molar, m. 3 (fig. 7), is elongated, narrowed and round- 

 ed posteriorly, scarcely emarginate on the sides ; it has two large 

 transverse lobes, as in the preceding, a very obsolete anterior basal 

 margin, and a large posterior undivided lobe, acute at the top, and 

 almost as much elevated as the two principal lobes. This lobe is 

 separated from the second lobe by a valley, acute at the bottom, 

 and a little deeper iaternally than externally ; into which fits the 

 small posterior cusp of the third superior molar. The valley sep- 

 arating the second from the first lobe is wide, and deeper inter- 

 nally. At the outer part it has a small horizontal triangular face (a) ; 

 and the external margin of this face forms with the anterior lobe 

 a very distinct obtuse angle : with the second lobe it forms a less 

 distinct right angle. It is to be observed that the internal extrem- 

 ities of the transverse lobes of these lower molars are more elevated 

 than the external parts. 



