1910.] The American Edentates. 341 



does not accept the Ganodonta as ancestral Xenarthra (1905.2, pp. 230- 

 235). 



In case the Xenarthra have been derived from Placentals having a dental 

 formula of g;};^;! (and the retention of a milk dentition in Tatusia and of 

 front teeth in the Santa Cruz armadillo PeltcpJiilus rather strengthen this 

 supposition), the most primitive Ganodont genera Onijchodectes and Con- 

 oryctes would serve to illustrate the probable characters of the ancestral 

 Xenarthra. These genera resemble the Creodonts and primitive Insecti- 

 vores in having a long low sagittal crest, a small brain case, a narrow post- 

 orbital region and heavy face, and teeth of four kinds, llie upper molars 

 are ovate-triangular, the lower molars of tuberculo sectorial derivation. In 

 case the Ganodonta should prove to have been ancestral to the Xenarthra 

 the ultimate derivation of the latter from forms with simple tritubercidar 

 upper and tuberculo-sectorial lower molars would be almost demonstrated 

 {cf. Osborn, 1907, 154-6). But even if the Ganodonta be not ancestral, 

 any evidence tending to show that the Xenarthra at one time had a dental for- 

 mula of l^fll would tend also to indicate that their upper molars were once 

 tritubercular; because many other groups in which the formula is derived 

 from |;^j;|;i| have also passed through the tritubercular stage. 



Resemblances to the Insectivora are usually obscured ,by marked differ- 

 ence in habits, but Peltephilus suggests the primitive Insectivore type in its 

 projecting muzzle, overhanging upper incisors, and low mandibular condyle, 

 while certain of the primitive mammalian characters of the Xenarthra are 

 preserved also among the Insectivora (e. g., low brain, traces of scales). 

 In possessing a well developed entotympanic (van Kampen, 1905, p. 498) 

 the Xenarthra recall the Menotyphla and Carnivora. 



Resemblances to the Rodentia also appears to be due largely to the 

 retention of- primitive Placental characters. The astragalus of certain 

 Miocene Armadillos (Scott, 1905, pll. xiv) strongly suggests the Rodent 

 type. 



Conclusion. The balance of evidence appears to indicate that the 

 existing Xenarthra, like the Tubulidentata, the Pholidota and perhaps the 

 Rodentia, are the highly modified descendants of some Mesozoic Placentals, 

 which branched off before the differentiation of Insectivores, Creodonts and 

 Protungulates. 



