THE NAUTILUS. 113 



A NOTE UPON THE INSUFFICIENCY OF THE OPERCULUM AS A BASIS 

 OF CLASSIFICATION IN ROUND-MOUTHED SHELLS. 



( Concluded) . 



BY L. P. GRATACAP. 



SECOND SERIES. 



Operculum subcircular, few whorls, rather quickly widening, nucleus sub- 

 central. 



I. Operculum horny. 



Genera, Jamaicia, Licina, Choanopoma. 



Jamaicia. Operculum horny, exteriorly convex, with few rugose whorls, 

 obliquely striated, sublammellose. 



Licina. Operculum not described. 



Choanopoma. Operculum horny, subcircular, whorls rather quickly widen- 

 ing, with free external margin, acute, frequently raised into lofty lamella, 

 nucleus subeccentric. 



II. Operculum cartilaginous. 



Genus, Adamsiella. 



Adamsiella. Operculum circular, thin, subcartilaginous, with few whorls, 

 gradually increasing, with somewhat free edges, nucleus subcentral. 



THIRD SERIES. 



Operculum oval or angulated, few whorls, more or less quickly increasing, 

 nucleus eccentric. 



I. Operculum shelly. 



Genera, Lithidion, Otopoma, Cyclostomus, Tudora, Leonia. 



Lithidion. Operculum subcircular, shelly, whorls rather rapidly increasing, 

 with a subcentral keel, strong, convex, furnished with simple margins. 



Otopoma. Operculum horny, solid, few whorls, convex center, simple 

 margin. 



Cyctoslomus. Operculum suboval, horny, plane, four to five whorls, gradu- 

 ally increasing, simple margin, nucleus subeccentric. 



Tudora. Operculum oval, horny, plane, two or three whorls rapidly increas- 

 ing, obliquely arcuate, striate, or sulcate, nucleus quite eccentric, with the left 

 and lower margin closely fitting to the peristomes. 



Leonia. Operculum oval, shelly, externally perconvex, unispiral, nucleus 

 placed near the columellar margin. 



II. Operculum cartilaginous, covered externally with a thin shelly layer. 



Cistula. Operculum oval, thin, cartilaginous, externally furnished with a 

 thin horny layer, with a few whorls gradually increasing, margin generally 

 free, nucleus eccentric. 



