162 PROCEEDINGS OF THE NATIONAL MUSEUM Vol.89 



distance behind the peristome, and it becomes attenuated and extends 

 into the hollow axis of the shell, which it plugs. 



The type, U.S.N.M. No. 493415, comes from Ceiba del Agua, 

 Pinar del Rio Province. It has 3.3 whorls remaining and measures: 

 Length, 9.6 mm.; greater diameter, 5.2 mm.; lesser diameter, 4.7 mm. 



We have seen specimens of this species also from Artemisa. 



This species is easily distinguished from the others by its narrower 



outer peristome and by the almost absent spiral sculpture in the 



umbilicus. 



Genus OPISTHOSIPHON Dall 



1905. Opisthosiphon Dall, Proc. Malac. Soc. London, vol. 6, p. 209. 



The shell varies from broadly ovate through elongate-ovate to 

 cylindro-conic. The nuclear whorls are microscopically granulose. 

 The early postnuclear turns may be solute or appressed to the pre- 

 ceding whorl. Axial ribs are always present, varying in different 

 groups from slender, hairlike elements to lamellae; their spacing varies 

 widely in different groups. Fine microscopic axial threads may or 

 may not be present between the heavier ribs. The spiral sculpture 

 may consist of strong cords that may be present on all parts of the sur- 

 face, or it may be restricted to the umbilicus. The last whorl may be 

 solute or adnate to the preceding turn. The umbilicus presents a 

 wide range of variance. It may be narrow or wide, open or closed. 

 The aperture also presents considerable difference, varying from oval 

 to subcircular, with the peristome always double ; the inner peristome 

 may be slightly or somewhat exserted; the outer peristome ranges 

 from narrow to broadly expanded in different species and this expan- 

 sion may extend over the entire lip or it may characterize only part of 

 it. An auricle may or may not be present at the posterior angle. The 

 operculum has the whorls separated by a narrow, deep groove, which 

 on the last whorl constitutes the plain chondroid edge. The parts of 

 the whorls between this inner edge and the groove are crossed by 

 numerous, retractively curved, decidedly strongly raised lamellae sep- 

 arated by narrow spaces. Behind the aperture is the breathing siphon, 

 upon which the generic name is based. This tube communicates by a 

 puncture with the interior of the aperture near the posterior angle, 

 slightly behind the edge of the peristome. The siphonal tube is usually 

 directed upward and backward into the suture, though in a number of 

 species with closed umbUicus the tube does not communicate at once 

 with the free surface, but with a channel situated behind the broadly 

 expanded, adnate parietal peristome of the outer lip, which in turn 

 communicates with the hollow axis of the sheU, and through this with 

 the exterior through the decollated apex of the shell. 



Type: Opisthosiphon (Opisthosiphon) bahamense Shuttleworth. 



