LECTURES ON MOLLUSC A. 53 



the neck. Any one who will compare a living Cydostoma with a 

 Snail or a Periwinkle, (or their pictures,) will observe how unlike the 

 general shape of the body is to its air-breathing ally, and how similar 

 it is to the Sea-snail. The general resemblance is fully borne out by 

 the details. The Cydostoma has the eyes at the base of the tentacles, 

 a long snout, a spiral operculum, and teeth arranged in seven series, 

 3*1 '3, after the rasping fashion of the true herbivorous Rostrifers. 

 Moreover, the sexes are distinct, exhibiting a far higher type of 

 structure than in the hermaphrodite snails. 



The Cydostoma family are known, among land shells, by their 

 graceful shape, varying however from that of a PlanorUs to a Turn-' 

 tella, the whirls often scarcely touching, and ending in a round mouth. 

 They are very numerous, both in sectional forms and in species. Dr. 

 Gray divides them into thirty genera, principally on differences in the 

 form of the operculum and mouth. The following are the principal 

 groups. 



Cydostoma proper has a shelly, ovate operculum, of few whirls as 

 in Litorina. Tropidophora has the whirls somewhat flattened and 

 keeled. Otopoma has a ear-shaped excrescence partially covering the 

 umbilicus. In Tudora (a West Indian group) the mouth is pinched 

 at the top. Chondropoma has the operculum nearly horny. Clioano- 

 poma is a singularly beautiful group, abounding in Jamaica, with a 

 spreading, generally frilled, lip, and a raised operculum. Realia is a 

 small Litorina-shsLped. group from the islands of the Old World and 

 the Pacific, with thin horny operculum ; and Bourciera is a singular 

 shell from Ecuador, shaped like Helicina. In this group the sole of the 

 foot is grooved, and the animal progresses on each side alternately. 



In the Cydopliorus group, the shell is depressed, the epidermis 

 thick, and the operculum horny and many whirled. The tentacles 

 are long and pointed, and the foot broad, without groove. In Aulo- 

 poma, the operculum has a grooved border, fitting over the lip of the 

 shell. Leptopoma has the lip not complete, as in the snails. Diplom- 

 matina is pupiform ; and Alycceus has the last whirl curiously dis- 

 torted. So the fossil form Ferrussina has the mouth leaving the reg- 

 ular spiral, and turning upside down. 



In Craspedopoma, the operculum has two rims, one of which fits 

 within, the other outside the contracted mouth. Cydotus has a flat- 

 tened shell ; and the operculum has a shelly layer outside the horny 

 one. In Pterocydus, the operculum is tur retted, as in Torinia; and the 

 lip is produced into a roof-shaped beak at the suture. The form is 

 found in the East Indian Archipelago ; as also Opisthoporus, in which 

 a little tube comes out behind, as in Typliis. Megaloma has a cylin- 

 drical shell and horny operculum ; and Cataulus has the base keeled 

 round the pillar, with a horny, many-whirled opercujum, which can 

 be drawn down out like a cork-screw. 



The Pupince are a group of beautiful little glossy shells from the 

 East Indian Archipelago. The lip is notched, in front and at the 

 suture; and the operculum is thin, horny, and many-whirled. In 

 Pupinella, there is a rudimentary canal, twisted back. Rliegostoma 

 has the axis bjnt, as in Streptaxis; and in Callia there is a shining 

 deposit over tl e spire, as in the Marginellce. 



