39 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



The upper molars are somewhat more complex ; in unworn teeth the 

 crown is similarly composed of three oblique laminae, of which the median 

 one is the longest and the posterior the shortest, and the latter is subdi- 

 vided into two parts by a shallow, transverse groove ; of these two parts, 

 the inner one has a pocket-like depression of enamel on the posterior side. 

 In the condition of wear which characterizes the adult animal, and which 

 most specimens display, the crown of an upper molar is divided by a deep 

 internal fold into two triangular prisms, of which the 

 anterior one is considerably the larger ; the outer end 

 of the foremost valley soon becomes isolated as an 

 enamel lake, while the outer end of the posterior valley 

 is also separated from the inner end and remains for 

 some time as an external enamel fold, but in the fully 

 y ^ adult animal it is separated from the outer wall of 



Grinding teeth of the crown. The tooth then has a deep internal fold 

 Myocasior coy pus, and three enamel lakes. Of these the anterior one is 

 crown view, x{. a. originally a part of the deep inner fold, which has an 

 Right upper teet . o ^jjq ue course but its outer portion is soon separated 



b. Right lower teeth. . . 



to form a lake. A fourth very minute lake is formed by 

 the isolation of the enamel pocket, mentioned above as impressing the 

 posterior side of the hindmost lamina. This minute lake disappears at 

 quite an early stage, so that many teeth show no trace of it. Aged ani- 

 mals, in which the teeth are much worn, lose even the inner fold, which 

 is first converted into a lake and then obliterated. 



In the lower jaw the grinding teeth have a similar character, but are 

 reversed, the deep fold which divides the crown into two prisms being 

 external, and the posterior one of the three lakes is an isolated portion of 

 this fold. All the folds are somewhat shallower than in the upper teeth 

 and hence are sooner cut off from the peripheral wall by abrasion. All of 

 the grinding teeth diminish in diameter toward the base, so that their size 

 is reduced by wear ; especially is this true of m T , which is the first of the 

 permanent lower teeth to be erupted and in an advanced state of wear 

 appears to be quite disproportionately small. 



Milk Dentition. I have seen no example of the temporary incisors, which 

 are probably shed very early. On the other hand, the milk premolar is 

 frequently found, but in most cases is so much worn that its pattern is not 

 clearly displayed. Plate LXV, fig. 3, shows an isolated tooth which I 



