Eaton — Vertebrate Fossils from Ayusbamba, Peru. 147 



Ayusbamba specimen having a maximum diameter of 117 mm , 

 would require a minimum diameter of only 82 rnm , instead of 

 104 mra , as recorded. Tet its section, as shown in fig. 5, is still 

 very far from circular, and cannot be described by the same 

 terms that Pompeckj uses in reference to the tusks of M. 

 andium. The tusk of the latter species, figured by Ameghino,* 

 greatly exceeds the Ayusbamba specimen in diameter, and 

 possesses an enamel band relatively much narrower. Its curva- 

 ture and the angle of torsion of the enamel band are approxi- 

 mately the same as those of the Ayusbamba specimen. The 

 torsion of the tusk of M. bolivianus is not recorded in such a 

 way as to be available for comparison. 



Yiews of the grinding and labial surfaces of an unworn 

 molar from Ayusbamba are shown in Plate V, figures 3 and 4. 

 This tooth, imperfect anteriorly, is presumably a lower molar 

 of the left side. Large accessory tubercles rest like buttresses 

 against the outer columns, but the tubercles that flank the 

 inner columns, and those that lie in the depths of the inner 

 valleys, are so small as to be almost hidden beneath the thin 

 layer of cement. As a result of this arrangement, the tooth 

 would be characterized, during early stages of wear, by the 

 presence of trefoils on the outer columns only, while the inner 

 valleys would remain open. Although this tooth agrees, in 

 the foregoing characters, with the specific definitions of M. 

 andium given by Lydekkerf and by Ameghino in his work 

 already cited, the correspondence fails entirely when the con- 

 tours of the inner columns are considered. Lydekker states 

 that "The form of the dentine-disk on the columns which do 

 not bear trefoils is pear-shaped, with the apex directed toward 

 the adjacent column." The inner columns of the Ayusbamba 

 molar are as stout where they approach the median cleft as at 

 their lingual borders, and would not, at any stage of wear, 

 assume a pear-shaped outline. In view of this essential differ- 

 ence, I do not think that the Ayusbamba molar should be 

 referred to 21. andium. It compares much more closely with 

 the lower molar of M. bolivianus figured by Pom peck j4 The 

 small accessory tubercles that flank the inner columns and line 

 the inner valleys of this tooth find their counterparts half-con- 

 cealed beneath the cement layer of the Ayusbamba molar. 

 The transverse measurements of the tooth of If. bolivianus, 

 taken at the base of the crown, viz., for the 1st cross cusp 7"4 cm , 

 for the 2nd 7'75 cm , for the 3rd 8'0 cra , for the 4th 7'8, for the 

 5th 7'4 cm , correspond very nearly with the following transverse 

 measurements taken in the same way from the Ayusbamba 



*Mam. Fos. de la Repub. Argent., p. 640. 

 t Cat. Foss. Mam. Brit. Mus., 1886. 

 J0p. cit., Taf. Ill, Fig. la. 



