198 PROCEEDINGS OF THE NATIONAL MUSEUM Vol.89 



continuation of the axial ribs and marked within the umbilicus by 

 several whitish threads, which render the axial ribs feebly nodulose. 

 The umbilicus is moderately wide but is shielded largely by the 

 reflected peristome. Aperture broadly oval; peristome double, the 

 inner exserted and slightly reflected; the outer forming a conspicuous 

 auricle at the posterior angle, narrow on the basal half of the outer 

 lip and the basal lip, and broadly expanded on the inner lip, on the 

 middle of which it is somewhat inbent. On the parietal wall it forms 

 a broad flap, which is adnate to the preceding turn. The entire outer 

 peristome is marked by slender, constricted laminae. Operculum 

 typically opisthosiphonid. Siphon at the posterior angle reflected 

 backward into the suture. 



A large series of specimens were collected on Signal Hill and the 

 other hilltop on Tmiguan6 Island. 



The type, U.S.N.M. No. 535469, has 4.4 whorls remaining, and 

 measures: Length, 13.8 mm.; greater diameter, 7.8 mm.; lesser 

 diameter, 6.0 mm. 



CuBiTASiPHONA, new subgenus 



The members of this subgenus have axial ribs which at more or 

 less regular intervals are gathered into tufts or cusps at the summit. 

 No fine axial threads are present in the intercostal spaces. The 

 spiral sculpture is confined to the umbilicus. In some species there is 

 a tendency toward nodulation or vertebration of the axial ribs, partic- 

 ularly on the early whorls ; this condition lends a false appearance of 

 spiral threads, but in every such instance the intercostal spaces fail to 

 show even a trace of spiral sculpture. 



Type Opisthosiphon {Cubitasiphona) poeyi, new species. 



KEY TO THE SPECIES OF THE SUBGENUS CUBITASIPHONA 



Umbilicus closed. 



Axial ribs very fine poeyi 



Axial ribs not very fine. 



Inner peristome very strongly protracted protractum 



Inner peristome not very strongly protracted. 



Suture strongly constricted claudens 



Suture not strongly constricted guanajaense 



Umbilicus not closed. 



Last whorl adnate to preceding turn. 

 Axial ribs stout and coarse. 



Nodulation on axial ribs conspicuous quinti 



Nodulation on axial ribs absent. 



Middle of inner lip of outer peristome slightly 



notched manatiense 



Middle of inner lip of outer peristome not slightly 

 notched bioscai 



