278 ON THE EXTINCT MAMMALIA OF 



cimen, represented in figure 46, is four-fifths worn away. Its central lakes appear 

 narrow in comparison with those of the former specimen, in consequence of their 

 being nearly worn to the bottom. 



In both teeth no trace of a posterior valley to the crown exists, as it had appa- 

 rently long been obliterated. In the half-worn tooth the internal column of the 

 crown appears associated by a very contracted isthmus with the antero-median 

 column ; in the more worn specimen the isthmus alluded to is wide comparatively. 

 The measurements of the teeth are as follow : 



Lines. Lines. 



Length externally, . . . . . . 9i 4 J 



Length internally, . . . . . . Qi 3 



Antero-posterior diameter, . . . . .8 71 



Transverse diameter, . . . . . .9 9 



3. Three upper molars, apparently a fourth, and a right and left fifth of the series. 

 The former, represented in figure 45, is nearly half worn away; the latter, repre- 

 sented in figures 43, 44, are hardly a fourth worn away. 



In the fourth molar, the central lakes of the triturating surface in their wide 

 gaping character resemble closely those of the less worn of the two preceding speci- 

 mens described. The anterior lake is almost devoid of cementum, which appears not 

 to be the result of accident after the death of the animal to which the tooth belonged. 

 No trace of a posterior valley to the crown exists. 



The little worn fifth molars likewise exhibit the usual simplicity of arrangement 

 of enamel on the triturating surfaces, figures 43, 44, and the central lakes appear 

 capacious. In both specimens the posterior valley is seen to exist, but in neither 

 does it extend more than one-third the length of the crown, so that by the time the 

 teeth were half worn away it would have been obliterated, as in the preceding speci- 

 mens. In the less worn of the two teeth, the posterior median lake opens into the 

 posterior valley of the tooth. The measurements of the specimens are as follow : 



Length externally. 

 Length internally, 

 Antero-posterior diameter, 

 Transverse diameter, 



4. A last superior molar, apparently two-thirds worn away, and represented in 

 figure 39. It agrees with the preceding, and the corresponding tooth of P. perditus, 

 in the simplicity of arrangement of the enamel, and the gaping character of the 

 central lakes of the triturating surface. Its measurements are as follow : 



