146 Eaton — Vertebrate Fossils from Ayusbamba, Peru. 



Most of the mastodon bones found at Ayusbamba present 

 no characters that can be utilized in the identification of the 

 species. As might be expected, it is best to rely principally 

 upon the form and structure of the teeth. Two specimens 

 have therefore been selected to illustrate the type of dentition. 

 From the piece of a right (upper) tusk shown in Plate V, 

 figures 1 and 2, the following measurements have been taken : 



Circumference of proximal end 354 mm 



Breadth of enamel baud.. 90 



Maximum diameter at proximal end 1 1 7 



Minimum diameter at proximal end 104 



Angle of torsion. . 12° 



The last of these measurements requires a few words of 

 explanation. A mastodon's tusk upon which the enamel band, 

 or other line of growth, describes a helix, presents a certain 

 analogy to a screw; and the torsion of such a tusk may 

 properly be regarded as a pitch angle. The angle of torsion, 

 used here, is defined as the angle that a tangent to the spiral 

 enamel band, at any point, makes with the adjacent axial line 

 of the tusk. This measurement is easily taken. It has the 

 further advantage of being independent of length, diameter 

 and circumference ; and may prove to be of considerable 

 taxonomic value. The mere statement that an enamel band 

 makes one revolution, or one-half revolution, about the axis, 

 without any record of length and diameter, does not adequately 

 describe the torsion. 



The form of the tusk of M. bolivianus is described by Pom- 

 peckj* as follows : " Der Querschnitt ist vollkommen elliptisch ; 

 die Durchmesser sind an der hinteren Bruchflache 8*9 : 6"2 cm , 

 an der vordere 6*2 : l - 7 em . Das z. T. abgefallene Schmelzband 

 hat vorne eine Breite von 5"65 m ." In another placef he com- 

 pares this tusk with that of M. andium : u Die schlanken, sehr 

 wenig gekrummten, in Schraubenspirale von ungefahr 1/2 

 "Win dung gedrehten Stosszahne haben bei Mastodon bolivianus 

 stark komprimierten elliptischen Querschnitt, nicht den kreis- 

 runden der Zahne von Mastodon Andium? 3 



That the enamel band is very broad in M. bolivianus is evi- 

 dent from Pompeckj's measurements of the anterior portion of 

 a tusk and also from his illustration of a basal fragment,;}: where 

 the enamel appears to cover one-third of the circumference. 

 The band is a trifle narrower in the fragment of tusk from 

 Ayusbamba, which differs also from M. bolivianus in the ratio 

 of the maximum and minimum diameters. To comply with 

 the diametric ratio (8*9 : 6'2) obtained from M. bolivianus, the 



' * Op. cit., p. 38. f Op. cit., p. 44. 



\ Op. cit,, Taf. IV, Fig. 56. 



