OF MOLLUSCA. 79 



Rossmiisler has examined the teeth of Cyclostomus elegans, C. 

 sulcatus, C. (Leonid) ferrugineus, and C. (Tudora) mamillaris. — 

 Icon. iii. 28. 



A. Operculum orbicular, often concave externally, of many very 

 gradually enlarging whorls ; nucleus central. 



a. Cyclotina. Operculum thick, formed of two lamina, with a groove 

 on the edge between them, the interior horny, the outer layer 

 calcareous ; the whorls usually furnished with a raised border 

 on the outer edge, forming a spiral ridge or fringe. 



1. Cyclotus. 

 Operculum thick, testaceous, outer surface rather concave ; whorls 

 numerous, very gradually enlarging, furnished with a thickened or 

 raised border on the outer edge. Shell subturbinate, depressed or 

 discoidal. Peristome continuous or joined by a callous margin, simple 

 or double, straight or reflexed. 



1. C. Inca, t. 311. 



2. C. ? trochiformis, t. 303. f. 11. 



3. C. ? anguliferus, t. 303. f. 9. 



4. C.I maculatus, t. 303. f. 7. 



2. Pterocyclos. 



Operculum subcartilaginous, many-whorled, spirally lamellated, 

 concave inside. Shell broadly umbilicated, rather discoidal. Peri- 

 stome generally double ; the internal short, cut on the right margin ; 

 the external dilated above into a roof-shaped beak. 



1 . P. rupestris. 



3. Opisthoporus. 



Operculum calcareous, thick, many-whorled, formed of two laminae 

 with an intermediate hollow space and a groove round the exterior 

 margin. Shell depressed, orbicular, broadly umbilicated ; last whorl 

 with a small, sutural, exserted tube behind the aperture. Peristome 

 double, expanded. 



1. 0. biciliatus. 



2. O. tubuliferus. 



4. Craspedopoma. 



Operculum horny, solid ; whorls very narrow, with a central nu- 

 cleus ; external plate flat ; internal deeply concave, furnished with a 

 circular prominence on the penult whorl. Shell subturbinate, gaping ; 

 last whorl a little constricted anteriorly. Peristome continuous, 

 simple, receiving the internal prominence of the operculum. 



1. C. lucidum. 



