136 PROCEEDINGS OF THE ACADEMY OF [Feb., 



Section ALBINULA ' Ckll ' Sterki. 

 Bifidaria pilsbryana Sterki. 



Rustler's Park; White Tail Canyon; Pine Canyon, 7,500 feet; Box 

 in Rucker Canyon; Limestone Mountain, in Shake Gulch. 



Section IMMERSIDENS Pils. and Van. 



In the central sierras, from the Grand Canyon of the Colorado to 

 the Mexican State of Michoacan, a distance of over 1200 miles, the 

 typical (or procera) group of Bifidaria gives place to a great extent 

 to a group in which the angular and parietal lamella? are less united, 

 joined only at the inner end of the angular lamella. The basal fold, 

 when present, is radial, or transverse to the cavity. This group is 

 morphologically in a more primitive stage than the eastern Bifidariae, 

 in which the angular and parietal lamellae are more completely con- 

 crescent. Some of the species, however, have evolved very com- 

 plicated apertures. Such forms as B. perversa and B. ashmuni are 

 to be viewed as highly evolved members of a low group. Immer- 

 sidens stands intermediate between Bifideria s. str. and Sterkia. In 

 the latter group the angular and parietal lamellae are not united. 



The following forms from Arizona and New Mexico belong here: 



Bifidaria perversa Sterki. 



" ashmuni Sterki. 



" ashmuni minor Sterki. 



" cochisensis Pils. and Ferr. 



" cochisensis oligobasodon Pils. and Ferr. 



" dalliana Sterki. 



" bilamellata Sterki and Clapp. 



Bifidaria prototypus Pils., from Michoacan, Mexico, also belongs to 

 the group, as a terminal member of a form-chain composed of ashmuni, 

 cochisensis, oligobasodon, prototypus, which constitute successively 

 simpler links in the chain. Probably the great unknown mountain 

 region of Northern Mexico will supply other members of the group. 



In order to complete the account of B. ashmuni and its allies, forms 

 from the Huachuca Mountains and elsewhere have been described 

 and illustrated below. 



Key to Species of Immersidens. 



a. — Shell sinistral, last whorl straightened and produced forward 



(fig. 29) B. perversa Sterki. 



a 1 . — Shell dextral. 



