1916] Merriam: Relationship of Equus to PUohippus 



533 



of the Equus forms of this continent were derived from PUohippus 

 along a line of evolution passing through or near the P. proversus 

 group. 8 Whether, as has been held by several writers, certain Old 

 World forms included in Equus had an independent origin by way of 

 Hipparion, may still be a field for discussion. Equus sivalensis and 

 E. namadicus reported from the uppermost Siwalik beds of India, 9 

 are held to represent true Equus, and have been considered as possible 

 derivatives from Hip pari on. Much remains to be known concerning 

 the characters of these forms, as also concerning their exact occur- 

 rence in the geologic scale. Should the uppermost Siwalik fauna con- 

 taining species referred to Equus prove to be late Pliocene there would 

 seem to be abundant opportunity for origin of the Indian forms from 

 an American type of the geologic stage represented by P. proversus, as 

 the upper Etchegoin beds lie below the great Tulare section, which has 

 been presumed to be mainly, if not entirely, Pliocene. 



s The line of evolution from PUohippus to Equus has been recognized by 

 many writers, although the stages of transition have not been fully indicated. 



9 See Lydekker, Richard, Palaeontologia Indica, ser. 10, vol. 2, part 3, 

 pp. 21 to 30, pis. 14 and 15, 1882; also Pilgrim, Guy E., Records Geol. Surv. 

 India, vol. 43, part 4, p. 324, 1913. 



