212 Mr. Gray's Conckological Observations. 



the AmpulIaricE, where it is shelly. In the Zoophagous Cteno- 

 brancliia, or the channel-mouthed spiral shells, the nucleus of the 

 Operculum (which in these is always horny,) is near one end, but 

 the annular structure still continues to be exhibited. In some few 

 of these, the nucleus, which is near the point, begins to be 

 slightly incurved, thus leading on to the other group, or the Spiral 

 Opercula which are analogous to the spiral shells, increasing like 

 them by the deposition of the new matter on the edge of the last 

 whorle. This sort of Operculu7n is common to all the Phytophagous 

 Ctenobranchia, or the entire-mouthed spiral shells, as the Tur- 

 binldae, Cerithiadae, Melaniae ; and the CyclostomadjB. The Oper- 

 culum is commonly attached to the animal alone ; but sometimes, 

 as in Neritina, it is articulated to the columella. It varies also 

 in its substance, which is either horny or shelly ; in the number of 

 its whorles; and in its size with regard to the mouth; and these 

 variations aiford much assistance in the formation of natural groups. 

 The Opercula are used medicinally on the continent, and were 

 used by our physicians, for they are contained in the old Pharma- 

 copoeias. The true Blatta Bi/zcmtina, or Sweet hoof, is the 

 Operculum of the Pteroceras, which somewhat resembles the claw 

 of an animal. The false Umbilicus Marinus^ or Sea navel (for 

 the Acetabularia is the true,) is the large shelly operculum of 

 a Tnrbo ; and the small shelly ones are used at present in the 

 Island of Guernsey under the name of ei/e-stones, to be placed on 

 the eyes of persons who have sand or gravel in their eyes, for the 

 purpose, as they say, of travelling round the eye and bringing 

 the sand out; but in fact they only separate the lid from the eye, 

 and thus allow it to clear itself by its continued motion. 



§ 6. On the Clausium. 



Of all the Avonderful contrivances employed by nature for the 

 protection of these animals, there is none Avhich is more calculated 

 to excite the admiration of the Conchologist than the Clausiuvi, 

 an elastic appendage which closes the aperture of the Clausilics^ a 

 genus of land Mollusca. It consists of a spirally twisted, thin, 

 shelly plate, inclosed in the last whorle of the shell, and attached 

 to the columella by an elastic pedicle. When the animal is re- 



