THE NAUTILUS. 127 



back, somewhat flattened over the palate, with a slight to strong, not 

 sharp, crest behind the margin, with two spiral impressions, one over 

 the lower palatal and another near the base; aperture broadly ellip- 

 tical to almost circular, peristome continuous or its ends closely 

 approximate and connected by a raised callus, well everted, with a 

 slight to rather strong lip thickening; lamellae and plicae, parietal and 

 angular, well differentiated, connected, large, the angular connecting 

 with the peristome; columellar complex, with a lower axial and an 

 upper horizontal part, " basal " a short, transverse lamella, rather 

 abrupt; lower palatal far remote from the margin, but visible in front 

 view, rather long, lamellar, thin, upper palatal somewhat less deep- 

 seated, close to the lower, somewhat oblique, shorter. 



Alt. 2-2.4, average 2.2, diam. 0.9 mm.; aperture alt. 0.8 mm. 

 Soft parts not seen. 



Habitat: Foothills of Plumosa Range, about 8 miles east of 

 Quartzsite, Yuma county, Arizona, in drift, in company with B. 

 hordeacella Pils. and B. tuba intuscostata Clapp, discovered by Mr. 

 Geo. H. Clapp, who states that the ratio of bilamellata and horde- 

 acella, in the drift, was almost exactly 1:16. Large numbers of 

 both species were found. 



B. bilamellata is very distinct and different from all other Bifi- 

 daria. At first sight it might be taken for hordeacella for its size 

 and shape, but a glance at the aperture is sufficient to recognize it, 

 and also the posterior aspect is different. With respect to the peri- 

 stome it approaches B. ashmuni; the parieto-angular lamella is of the 

 same formation, and the palatal plicae are similar but not so deep- 

 seated; the "basal" is the same as in B. dattiana. The n. sp. is 

 very interesting by the combination of features of apparently widely 

 different species, and appears to be intermediate between two groups. 



EXPLANATION OF PLATK VIII. 



Figs. 1, 2, 3. Ashmunella kochii Clapp. Type, No. 5765, coll. 

 G. H. Clapp. Description on p. 77. 



Fig. 4. Bifidaria clappi Sterki. Cotype, Knoxville, Tenn., No. 

 98279, A. N. S. P. Description on p. 108. 



Figs. 5, 6. Holospira bartschi P. & C. Two cotypes, coll. A. N. 

 S. P. and G. H. Clapp. Description on p. 114. 



Fig. 7. Bifidaria bilamellata Sterki & Clapp. Front view of a 

 cotype, No. 98268, A. N. S. P. Description on p. 126. 



