i9o8.] SINCLAIR— THE SANTA CRUZ TYPOTHERIA. 65 



at Princeton University and the American ]\Iuseum of Natural His- 

 tory it has been found impossible to separate in a satisfactory manner 

 the species of the genus Protypothcrinm. An almost exact inter- 

 gradation in size without appreciable difference in structure is 

 observable between the largest species Protypotheriuni australe (Fig. 

 i) and the smallest P. attejuiatiim. As none of the collections have 

 been made with strict regard to stratigraphic sequence, we are not 

 in a position to say whether these differences represent individual 

 variations or true mutations. The former alternative has been 

 adopted in monographing the group (see the forthcoming Volume 

 VI., Part I. of the " Reports of the Princeton University Expeditions 

 to Patagonia "). 



Classification of the Santa Cruz Typotheria. 



The Typotheria are grouped by Scott- as a suborder of the 

 Toxodontia and may be defined as follows : 



Plantigrade or digitigrade mammals with pentadactyF or tetra- 

 dactyl feet, strongly interlocking carpus with os centrale and serial 

 or slightly interlocking tarsus with hemispherical astragalar head.- 

 Dentition usually complete but tending toward reduction of the 

 lateral incisors, canine and anterior premolars in specialized forms. 

 Median incisors more or less enlarged and functional as cropping 

 teeth. Molars hypsodont, lophoselenodont in crown pattern, curving 

 inward above and outward below. A clavicle is present in some 

 forms. Femur with third trochanter. Fibula articulating with 

 calcaneum. 



Two well-marked families are recognizable among the Santa 

 Cruz representatives of the suborder for which the names Intera- 

 theridse and Hegetotheridae have priority. Each contains a large 

 and a small genus of which, in either case, the former is the less 

 specialized. The following key to the families and genera may 

 facilitate the determination of new material : 



^ Scott, W. B., " The Miocene Ungulata of Patagonia," Rcpt. British 

 Asso. Adv. Sci., 1904, pp. 589-590. 



^ Ameghino figures a pentadactyl manus in Pachyrukhos typicus, " Contrib. 

 al conocimiento de los mamiferos fosiles de la Repiiblica Argentina," Adas 

 de la Academia Nacional de Ciencias en Cordoba, T. V., PI. 13, fig. 14, 1889, 

 and in Typotherium, ibid., PI. 18, fig. 5. 



