no PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



chiefly affecting the posterior half of the crown. The postero-external 

 cusp is much smaller than the antero-external and is no longer in the 

 same fore-and-aft plane with it, but is so inclined as to present backward 

 rather than outward. 



The roots of all the upper molars are peculiar ; there are two external 

 roots of the usual character, and one very large internal root, which is 

 laterally compressed, but extends through the whole antero-posterior 

 length of the crown, and shows distinct indications of being formed by 

 the coalescence of two separate roots, of which the anterior is much the 

 larger. The coalescence is by means of a thin sheet of dentine and cement, 

 the two nerve-canals remaining separate. 



In partially worn examples of the upper molars the various elements 

 of the crown-pattern can no longer be distinguished ; the longitudinal 

 valley and the anterior and posterior fossettes remain as enamel lakes. 

 In more advanced stages of attrition the anterior part of the valley is worn 

 away and only two lakes are left, except in those cases in which a third 

 small lake is enclosed between the two internal cusps and the cingulum. 

 In very old individuals, of course, the lakes and all other elements disap- 

 pear, with the exception of the surrounding wall of enamel. 



B. Lower Jaw. The lower teeth have somewhat less complex patterns 

 than the upper. The first seven teeth, from ir to ps inclusive, form a series 

 of similar shape, increasing gradually in size and complexity backward. 

 The incisors are simple, compressed conical teeth, with crowns which 

 resemble those of the upper jaw in shape, but are somewhat lower and 

 broader and have a better developed internal cingulum. The median 

 incisors of the two halves of the mandible are almost in contact and bite 

 between the two corresponding upper teeth. The first and second incisors, 

 and sometimes the third also, are so inserted in the jaw that their principal 

 diameter is transverse. The canine is like the incisors, except in being 

 larger. 



The first lower premolar may be described as an enlarged canine, and 

 is single-rooted ; p2, which is inserted by two roots, is still of the same 

 general form, though of larger size, and has a much more prominent inter- 

 nal cingulum, which encloses small anterior and posterior basal fossettes. 

 The third premolar is much more elongated antero-posteriorly than pa 

 and shows the beginning of the bicrescentic pattern. Externally the crown 

 has but a single, undivided cusp, but on the inner side, a prominent 



