134 PROCEEDINGS OF THE NATIONAL MUSEUM Vol.89 



Breathing siphon absent. 



Breathing puncture present. 



Breathing pore communicating with the space behind 



the withdrawn operculum Xenoporaoides 



Breathing pore communicating with a channel at the 

 posterior angle of the aperture connecting with the 



hollow axis somewhere behind the peristome 



Rhytidothyra 

 Breathing device absent. 



Last whorl of operculum covered by an overarching dome Xenopoma 



Last whorl of operculum not covered by an overarching dome.. 



Parachondria 



OPISTHOCOELICUM, new genus 



In this genus the operculum resembles that of Rhytidothyra, i. e., 

 the reenforcements on the outside consist of strongly raised, retrac- 

 tively curved lamellar ribs, which extend completely across the turns 

 (they are not separated by a channel as they are in Opisthosiphon). 

 A siphon is present immediately behind the peristome, at the posterior 

 angle of the aperture. This siphon bends into the suture, and it 

 connects with the umbilicus by a channel behind the expanded outer 

 peristome. 



Type. — Opisthocoelicum (Opisthocoelicum) opisthocoele, new species. 



The genus appears to be confined to the provinces of Santa Clara 

 and Camagiiey. 



We are recognizing three subgenera, which the following key will 

 help to distinguish: 



KEY TO THE SUBGENERA OF OPISTHOCOELICUM 



Operculum too large to be withdrawn into the shell Opisthocoelex 



Operculum not too large to be withdrawn into the shell. 



Axial ribs gathered into hollow tufts at the summit Opisthocoelops 



Axial ribs not gathered into hollow tufts at the summit 



Opisthocoelicum 



Opisthocoelex, new subgenus 



In this subgenus the inner peristome is much exserted and the oper- 

 culum is much too large to be withdrawn into the shell, which it 

 merely caps. The retractively curved lamellae on the outside of the 

 operculum are fused laterally to form an almost continuous plate, 

 resembling somewhat the operculum of Colohostylus of Jamaica. 

 The breathing device is the same as in typical Opisthocoelicum. In 

 the present subgenus the axial ribs are thickened at the summit, 

 but they are not gathered into tufts. 



Type: Opisthocoelicum (Opisthocoelex) paradoxum (Torre and 

 Henderson) . 



