THE WEST MffiEICAN MOLLUSKS OF THE GENUS 

 AI^lPHITHAEA^rUS. 



By Paul Bartsch, 



Assistant Curator, Division of MoUushs, U. S. National Miiseum. 



The genus AmpMihalainus was established by Dr. P. P. Carpenter 

 in 1SG5 ^ for Amyhitlialamus indusus Carpenter. Watson's name 

 Scrobs ' is a synonym of it. In 1S66 Carpenter described another 

 species, AmpMthalamus lacunatus Carpenter.^ 



To these I now add a third, Ampliithal'amus tenuis, also from 

 California. 



Of these three, A. indusus Carpenter has a peripheral cord; A. 

 tenuis Bartsch, a faint raised line only; A. indusus Carpenter is 

 broadly conic; A. tenuis Bartsch, elongate ovate. In A. lacunatus 

 Carpenter there is no peripheral cord. 



AMPHITHALAMUS LACUNATUS Carpenter. 



Amphithalamus lacunatus Carpenter, Proc. California Acad. Nat. Sci., vol. 3, 

 1866, p. 218. 



Shell minute, broadly conic, milk-white. Nuclear whorls one and 

 one-half, well rounded, marked by about 15 slender, 

 equal and equally spaced, spiral threads and nu- 

 merous, very fine, axial threads, lending the sur- 

 face a very minutely pitted appearance. Post- 

 nuclear whorls feebly shouldered at the summit, 

 slightly rounded, marked b}' incremental lines only. 

 Suture well impressed. Periphery of the last whorl 

 inflated, rounded. Base moderately long, slightly 

 rounded, with a narrow umbilical chink. Aperture 

 incomplete (outer lip fractured) ; inner lip curved, 

 appressed; parietal wall covered with a thick callus, fig. i.— amphithala- 



Doctor Carpenter's type (Cat. No. 15564, ^Z''^'^'^^'"'" '''"^' 

 U.S.N.M.) has two post-nuclear whorls and meas- 

 res: Length 1. 5mm., diameter 1.1 mm. It comes from shell wash- 

 ings at San Pedro, California. 



1 Ann. Mag. Nat. Hist., ser. 3., vol. 15, p. 181. 



2 Challenger Report, 1886, p. 611. 



3 Proc. California Acad. Nat. Sci., vol. 3, 18G6, p. 218. 



Proceedings U. S. National Museum, Vol. 41— No. 1854. 



263 



