1910.] 



NATURAL SCIENCES OF PHILADELPHIA. 



137 



b. — Columella!" lamella running forward on the parietal wall in the 

 position of an infraparietal lamella; inner end of the parietal 

 lamella curving strongly toward the outer wall. 



c. — Length about 2 mm B. ashmuni Sterki. 



c 1 . — Length 1.6 to 1.8 mm B. a. minor Sterki. 



b 1 . — Columellar lamella normal in position, horizontal and con- 

 spicuous in a front view. 



c. — Parietal barrier /-shaped, the inner end curving more or 

 less towards the outer wall. 

 d. — Basal fold well-developed, radial. 



B. cochisensis P. and F. 

 d 1 . — Basal fold minute; angular lamella reduced. 



B. c. oligobasodon P. and F. 

 d 2 . — Basal fold wanting; angular lamella reduced. 



B. prototypus Pils. 

 c 1 . — Parietal lamella straight or bending slightly toward the 

 columella at its inner end; shell small, rather slender. 

 d. — Length 2, diam. 0.85 mm., or smaller; nearly cylindric. 



B. dalliana Sterki. 

 d 1 . — Length 2 to 2.4, diam. 0.9; spire tapering more. 



B. bilamellata S. and C. 29 



Bifldaria perversa Sterki. Fig. 29. 



Bifidaria perversa Sterki, Nautilus, XII, December, 1898, p. 90 (Nogales). 



Found in the Chiricahuas in the Valley of Turkey Creek below 

 Paradise and in White Tail Canyon. Here- 

 tofore known only from Nogales and the drift 

 debris of the San Pedro River at Benson, 

 Arizona. 



Bifidaria ashmuni Sterki. Fig. 30, a, b, c. 



Bifidaria ashmuni Sterki, Nautilus, XII, Sept., 

 1898, p. 49 (Santa Rita Mountains, Arizona; 

 Cook's Peak and Dripping Springs, Organ Moun- 

 tains, New Mexico). 



Bifidaria ashmuni form minor Sterki, t.c, Dec, 

 1898, p. 92 (Nogales, Arizona). 



The typical form of B. ashmuni is nearly 

 cylindric, tapering but slightly, composed of 

 five strongly convex whorls. The last whorl 

 becomes straightened and slightly sinuous in 

 basal view, and has a more or less conspicuous 

 oblique swelling or crest some distance behind 



the aperture. This crest is not always so strong, as shown in fig. 30 c 

 (a specimen from Page's Ranch, Oak Creek, in central Arizona). The 



Fig. 29. — B. perversa, 

 basal view, most of 

 the basal wall re- 

 moved to show the 

 angular, parietal and 

 an d columellar 

 lamellae. 



29 Bifidaria bilamellata Sterki and Clapp, Nautilus, XXII, pi. 8, fig. 7 (March, 

 1909). Sterki, Nautilus, XXII, p. 126 (April, 1909). Yuma County, Arizona. 



