140 PROCEEDINGS OF THE NATIONAL MUSEUM Vol. 89 



is broadly oval; peristome double, the outer broadly expanded, a little 

 more so on the inner lip than on the outer, widest at the umbilicus, 

 over which it is reflected and broadly expanded on the parietal wall, 

 where it is adnate to the preceding turn; an inconspicuous auricle at 

 the posterior angle is made irregular by the siphon, which is reflected 

 backward, and which is appressed to the suture; the inner peristome 

 is slightly exserted and slightly expanded. Operculum paucispiral, 

 with the nucleus halfway between submarginal and subcentral; the 

 turns are marked by retractively slanting, slender lameUae, which are 

 fused on the outer border as well as on the inner one. 



The specimen described and figured, U.S.N. M. No. 355467, is one of 

 three received from Poey, collected by Gundlach at Nuevitas. It has 

 a little over 4 whorls and measures: Length, 13.6 mm.; greater diam- 

 eter, 7.2 mm. ; lesser diameter, 5.8 mm. 



The much flatter whorls and the weak auriculation at the posterior 

 angle of the aperture will readily distinguish this from Opisthocoelicum 

 (Opisthocoelops) occultum (Torre and Henderson) . 



Opisthocoelicum, new subgenus 



This subgenus is distinguished from the subgenus Opisthocoelex by 

 having the inner peristome scarcely at all exserted and by having the 

 operculum withdrawable into the aperture, with the lamellae distinct. 

 The umbilicus in adult shells is closed by the reflection of the outer 

 peristome of the inner lip. When the operculum closes the shell, 

 breathing may be accomplished through the puncture at the posterior 

 angle of the aperture, which communicates with the hollow siphon, 

 and which in turn bends into the suture, where it connects with a 

 small channel behind the expanded outer peristome of the parietal 

 wall, which leads to the umbilicus; the hollow axis and the narrow 

 opening at the decollated apex furnish contact with the exterior. 



Type : Opisthocoelicum (Opisthocoelicum) opisthocoele, new species. 



KEY TO THE SPECIES OF THE SUBGENUS OPISTHOCOELICUM 



Axial ribs closely spaced opisthocoele 



Axial distantly spaced laraellicostatura 



OPISTHOCOELICUM (OPISTHOCOELICUM) OPISTHOCOELE, new species 



Plate 9, Figure 8 



Shell moderately large, ovate-conic, ranging from white to pale 

 wax yellow. Nuclear whorls 2, decidedly inflated, strongly rounded, 

 microscopically granulose, forming an almost truncated mammillated 

 apex. Postnuclear whorls well rounded, the early ones marked by 

 rather distantly spaced, lamellar axial riblets, which become expanded 

 at the summit and suture; where summit and suture meet the riblets 



