.1910.] 



NATURAL SCIENCES OF PHILADELPHIA. 



141 



fold is very small. The upper margin of the peristome is adnate 

 for some distance. In this form, or race, if such it proves to be, 

 B. ashmuni cochisensis makes its nearest approach to B. dalliana and 

 to the following race. 



Possibly Dr. Sterki at the time of his original description had 

 cochisensis before him, together with typical ashmuni, since he mentions 

 specimens from the Santa Rita Mountains. We have examined a 

 lot collected by Mr. Ashmun at that place and find them all to be 

 .cochisensis. The terms of Sterki's original description apply only 

 to the form herein defined as ashmuni, for he mentions the crest 



Fig. 33. — Bifidaria cochisensis P. and F. Santa Rita Mountains, Arizona. 



"forming a projecting angle at the base" and the columella!- lamella 

 "ascending to the body-whorl between the parietal and columella." 

 Careful examination of a series of several thousand examples, supplying 

 data for the present account, has shown no intergrading forms between 

 ashmuni and cochisensis, yet careful cleaning of the aperture is neces- 

 sary for their discrimination. 



Bifidaria cochisensis oligobasodon n. subsp. Fig. 34, a, b, c. 



The shell is externally similar to B. cochisensis, but differs by the 

 reduction of all the teeth. The parietal barrier is simplified (fig. 34 b), 

 and the basal fold is reduced to a minute tubercle or a mere vestige, 

 or in a few apparently mature shells it seems to be wholly lost. Parietal 

 margin adnate. 



