80 SYSTEMATIC ARRANGEMENT 



b. Operculum horny, thin, without any external calcareous layer. 

 Shell conical, depressed; last whorl rounded, without any 

 groove or ridge round the front of the axis. 



5. Aulopoma. 



Operculum horny, with many whorls, Planorbis-shaped, composed 

 of two plaits, between which it has a spiral channel, which is inte- 

 riorly open on the last whorl. Shell turbinate-depressed, or dis- 

 coidal ; the last whorl disjoined anteriorly. Peristome free, straight, 

 continuous, immersed in the circular groove of the operculum. 



1. A. helicinum. 



6. Cyclophorus. 



Operculum horny, thin, closely whorled, more or less concave ex- 

 ternally. Shell globose-turbinate, depressed or discoidal. Peristome 

 continuous, expanded or straight. 



1 . C. volvulus. 



7. Leptopoma. 



Operculum membranous, closely whorled, flat. Shell globose-tur- 

 binate or conical, narrowly umbilicated. Peristome simple (rarely 

 double), reflexed (rarely straight), with distant margins, joined 

 sometimes by a very thin callus. 



1. L. multilabris, t. 311. f. 8. 



8. Alycleus. 



Operculum orbicular, rather shelly, obsoletely many-whorled. 

 Shell conical or depressed ; spire regular ; last whorl distorted, com- 

 pressed, much contracted behind the aperture ; mouth circular. 

 Peristome regularly reflected. 



1. A. gibbus. 



9. Diplommatina. 



Operculum shelly, thin, orbicular ; whorls few, with thin promi- 

 nent lamellae on their external edges. Shell scarcely rimate, thin, 

 subovate ; whorls convex, last subascendent ; aperture nearly cir- 

 cular. Peristome interrupted, expanded. 



1. D. folliculus. 



Mr. Benson denies that this genus has an operculum, and thinks 

 those described are adventitious ; they were attached to the animal 

 dried in the shell, and have been observed in two species. He also 

 describes "the eyes on the posterior part of the tentacles at their 

 base." Hence Mr. Adams has proposed it should be arranged with 

 the Truncal el lidce. 



