218 PROCEEDINGS OF THE NATIONAL MUSEUM Vol. 89 



bilicus, which it completely closes, and over the parietal wall. The 

 outer peristome is marked by concentric lamellae. Operculum 

 opisthosiphonid, that is, it bears numerous, closely spaced, decidedly 

 elevated, retractively slanting lamellae, which do not quite cover the 

 entire whorls but which leave the spaces free between adjacent whorls. 

 The outer and inner edges of these riblets are fused into a slender 

 lamella. The structure really more closely resembles early stages of 

 Xenopoma than Opisthosiphon. There is a breathing puncture at the 

 posterior angle of the aperture a httle behind the peristome. 



The type, U.S.N.M. No. 535537, was collected by Dr. Charles T. 

 Ramsden at Florida Blanca, Oriente Province. It has 4.6 whorls 

 remaining and measures: Length, 9.5 mm.; greater diameter. 5.6 

 mm.; lesser diameter, 4.3 mm. 



Genus RHYTIDOTHYRA Henderson and Bartsch 



1520. Rhytidothyra Henderson and Bartsch, Proc. U. S. Nat. Mus., vol. 58, p. 65. 



Shell elongate-ovate, marked by axial ribs and by spiral threads. 

 A breathing pore at the posterior angle of the aperture, a short dis- 

 tance within the peristome, communicates with a slender tube follow- 

 ing the posterior angle of the aperture on the inside for three or more 

 whorls, where it in turn communicates with a slender cleft in the 

 wall of the hollow axis, thi'ough which breathing may take place 

 through the narrow puncture at the decollated tip. The operculum 

 consists of many strongly raised, retractively curved lamellar ribs, 

 which extend completely across the turns. These lamellar ribs are 

 high at their inner edge, from which they pass down in a gentle ciu-ve, 

 rising again to their highest altitude at their outer border, where 

 they become fused into a spiral lamella, which projects almost as 

 far as the basal chondroid plate. The lateral margin of the operculum 

 is strongly channeled. 



Type: Rhytidothyra bilahiata (Orbigny). 



RHYTIDOTHYRA BILABIATA (Orbigny) 



Shell broadly conic, varying from comparatively large to moderately 

 small; ranging in color from ashy gray to reddish or even through 

 pale brown to rather bright red; never banded. The peristome varies 

 from white to pale orange; the inside of the aperture has the same 

 range of color variation. Nuclear whorls about 2, well rounded, 

 smooth, forming a small apex, which appears slightly truncated. 

 Postnuclear whorls strongly rounded, slightly shouldered at the 

 summit, marked by retractively slanting, slightly wavy, sublamellar, 

 closely spaced axial ribs, which are much more strongl}?- developed and 



