PROCEEDINGS OF THE NATIONAL MUSEUM. 369 



Genus APLEXA, Fleniing. 

 APLEXA HYPNORUM, Linnaeus. 



A single specimen of this species was found in the drift of the Santa 

 Cruz Eiver, near Tucson, Arizona, by Dr. Mearns. This is the most 

 southerly locality yet reported for this species, about the identification 

 of which there seems to be no doubt. The Santa Cruz is a tributary 

 of the Gila Eiver. 



Genus PLANORBIS, Guettard. 

 PLANORBIS LIEBMANNII, Dunker. 



A single specimen was received from 20 miles north of Campo, San 

 Diego County, California. 



PLANORBIS TUMIDUS, Pfeiffer. 



Specimens of this species were sent in by Dr. Mearns from Las 

 Moras Creek, Kinney County, Texas, and the Eio Grande Eiver, near 

 El Paso; from Seven Wells, the Santa Cruz Eiver near Tucson, San 

 Bernardino Eiver, and the Colorado Eiver at Yuma, Arizona; from the 

 San Bernardino Eiver and Sonoyta Eiver, northern Mexico, near the 

 boundary line; near monument '^o. 219; in Gardner's Laguna, Lower 

 California, and at Laguna Station, I^^ew Eiver, San Diego County, 

 California. 



This species varies enormously. Some of the specimens are less ele- 

 vated, with a wider umbilicus and one whorl more than others of the 

 same size, and taken separately would be regarded a.> distinct, but the 

 intermediate gradations are so numerous that I am unable to regard 

 them as forming a different species or even a really constant variety. 



Genus BYTHINELLA, Moquin Tandon. 



The following species has been kindly determined by Mr. Pilsbry, 

 who has made a special study of this genus. 



BYTHINELLA PALOMASENSIS, Pilsb-y. 



(Plate XXXI, fig. 9.) 



BytliineUa paJomasensis, Pilsbry, Nautilus, IX, p. 68, Oct., 1895. 



Shell small, ovate, rapidly tapering above from the periphery of the 

 body whorl to a blunt apex; composed of 4 very convex whorls, the 

 last about five-sixths the entire length of the shell, well rounded out; 

 surface showing only faint incremental lines; aperture ovate, subangu- 

 lar above, its longest axis about half the length of the shell; peri- 

 stome thin, continuous across the parietal wall and nearly straight 

 there, though not appressed to the body whorl ; umbilicus minutely per- 

 forate; color whitish corneous and somewhat translucent. Height of 

 shell 2.75, maximum diameter 1.80, longer axis of ai)erture 1.3G mm. 

 Proc. N. M. vol. xix 2^ 



