I 1 2 PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 



(deutero- and tetartocones), but the external median rib, or style, is less 

 prominent than in the true molars. 



B. Lower Jaw. The lower milk-teeth are more closely set than the 

 corresponding permanent ones and the first six form an imbricating series, 

 the hinder edge of each tooth overlapping the one behind it ; these teeth 

 all increase in size and complexity toward the posterior end of the series. 

 The milk-incisors and canines are lower, but relatively much more extended 

 antero-posteriorly than their permanent successors ; each has on its in- 

 ternal face a low, median, vertical ridge, which becomes more prominent 

 on each succeeding tooth and is most conspicuous on dpx and 2. 



Lydekker ('93, 68) states that in the mandible of young individuals of 

 T. lydekkeri the alveoli of the incisors are directed forward almost hori- 

 zontally. I am quite at a loss to understand this statement, for I have 

 seen nothing in the milk or permanent dentition which could justify, or 

 even suggest, such a description. On the contrary, in all of the individ- 

 uals examined by me the incisors, whether temporary or permanent, are 

 quite erect and show no tendency to become procumbent, and Ameghmo 

 ('94/5, 283) reports the same conclusion. I can only suppose that Mr. 

 Lydekker has had before him a specimen deformed by crushing, or pos- 

 sibly an abnormal individual 



All of the milk-premolars are implanted by two roots ; the third is im- 

 perfectly molariform, having two crescents and, in addition to the pillar 

 on the inner side of the posterior crescent which the molars also have, there 

 is a similar, though smaller, pillar in the concavity of the anterior crescent. 

 The fourth milk-premolar is indistinguishable from one of the permanent 

 molars, except for the presence of the pillar in the anterior crescent. 



The specimens at my disposal show little regarding the manner in 

 which the teeth succeed one another. The first molar in both jaws is in 

 function before any of the milk-teeth have been shed ; the second molar 

 is in process of eruption at the same time and was probably in use before 

 any of the second series. In one individual with milk-dentition still com- 

 plete, the fully formed, but as yet rootless, crown of p- is nearly ready to 

 displace dp-, and it therefore seems likely that that tooth is the first of 

 the second series to erupt. 



If we compare with the dentition of Theosodon that of Macrmichenia 

 patachonica, from the upper Pampean beds, the latest and most special- 

 ized member of the family, we observe that the changes are surprisingly 



