70 



SINCLAIR— THE SANTA CRUZ TYPOTHERIA. 



[April 24, 



two belong to the caudal series. The length of the tail seems to 



have varied. In Protypotherium and Interatherium it is both long 



and heavy while in PachyrnkJios there is 

 reason to believe that it was quite short. 

 4. Foot Structure. — Almost nothing 

 has hitherto been known of the struc- 

 ture of the feet in the Santa Cruz typo- 

 theres, but definite information is now 

 available for all the genera except He- 

 getotherhim, in which the manus is still 

 unknown, but from tlie close structural 

 resemblance of Hegetotherium and Pa- 

 chryiikhos it is probable that it was not 

 unlike that of the latter, which in turn 

 does not differ materially from the manus 

 of Interatherium and Protypotherium 

 (Fig. 6, A). In the Santa Cruz forms 

 both manus and pes are tetradactyl with- 

 out the slightest trace of an opposable 

 thumb or great toe.* The carpus is 

 strongly interlocking and shows no trace 

 of the centrale. Two types of hind foot 

 are developed (Figs. 5 and 7, A) simu- 

 lating the paraxonic and mesaxonic sym- 

 metry of the feet of the Artiodactyla 

 and Perissodactyla. These are probably 

 to be correlated in the Typotheria with 

 cursorial and saltatorial modes of pro- 

 gression. Pachyrukhos was certainly a 

 jumping animal as shown by the greater 



length and strength of the hind limbs and inner digits of the pes. 



In fact, the structure of both the fore and hind limbs in this animal 



^ A pentadactyl manus with separate centrale in the carpus and opposable 

 thumb and a pentadactyl pes with large opposable hallux figured by Ameghino, 

 Revista Argentina dc Hist. Nat., L, pp. 393, 394, figs. 95, 96 and referred to 

 Interatherium (Icochilus) robustum do not pertain to this genus. The same 

 figures with the erroneous determination appear also in Zittel's " Handbuch 

 der Palaeontologie," IV., p. 493, fig. 407. 



Ill 



Fig. 5. Left hind foot 

 of Protypotherium australe 

 Ameghino, three fourths 

 the natural size. (No. 

 9149 American Museum 

 of Natural History col- 

 lection.) 



