514 PROCEEDINGS OF THE ACADEMY OF [AugUSt, 



has kindly made a drawing of one of these. Dr. E. E. C. Stearns^ 

 has figured a number of specimens of Physa from the Colorado 

 desert; these are variously refei'red to humerosa, mexicana, etc. 



It would appear that the original hiivierosa was described from 

 fossil or subfossil shells, such as are common in the Southwest in 

 certain localities ; and fui'thermore, that the specimens taken as 

 typical represented a rather extreme variation. 



In the series from the Arroyo Pecos Pleistocene, some shells 

 agree well with the original humerosa, but these pass by insensible 

 gradations into forms which agree with what has been regarded at 

 the ]S;ational Museum and by Dr. Stearns as P. mexicana Philippi. 

 "Whether these latter are the genuine mexicana may only be deter- 

 mined when it is possible to study topotypes with the soft parts. 



It was also found that many shells from the Arroyo Pecos could 

 not be separated from the common Physa still living at Las Vegas> 

 though there were certain average differences between the living 

 and fossil series. The living species has recently been described by 

 Crandall - as P%.sa rhomboidea. Part of Craudall's original ma- 

 terial came from Las Vegas; he says the species " is distinguished 

 by its robust appearance, deep sutures, constricted aperture and 

 umbilicus, which will be found in a large part of them. It is more 

 like P. solida Philippi than any other species." It has impressed 

 spiral lines like P. gyrina, etc. 



For purposes of comparison large uumbei-s of P. rhomboidea 

 were collected from a pool in the Gallinas river at Las Vegas. The 

 variation in this series, all from the same place, was very great, 

 and selected specimens would readily be taken for distinct species. 

 However, upon close comparisons, no difference could be found in 

 the soft parts. Externally, the animals were the same ; internally, 

 there were some variations, as in the length of the oesophagus, but 

 these were not correlated with the differences in the shells, and were 

 clearly not of specific value. The teeth were of ihe same tyi>e 

 in all. 



In order to determine whether the teeth could be used in dis- 

 tinguishing Western species of Physa, I examined P. virgata Gould, 

 collected by Mr. Cockerell in Salt river, Tempe, Ariz., and a spe- 

 cies identical with what has been regarded at the J^ational Museum 



1 Proc. U. 8. Natl. Museum, XXIV, Pis. XXIII, XXIV. 

 'Nautilus, August, 19U1, p. 44. 



