136 PROCEEDINGS OF THE ACADEMY OF [Feb., 
Section ALBINULA ‘CkIl’ 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 lamelle 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 Bifidarie, 
in which the angular and parietal lamelle 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. IJmmer- 
sidens stands intermediate between Bifideria s. str. and Sterkia. In 
the latter group the angular and parietal lamelle 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 aterminal member of a form-chain composed of ashmunt, 
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 
(fier DO). taney cn fikee dade taeaee MeeMe eas Avan oan B. perversa Sterki. 
a',—Shell dextral. 
8 Se 
