LECTURES ON MOLLUSCA. 207 



often you cannot tread without crushing them. They live a sedentary 

 life,, adhering pretty firmly to the surface by their short, strong foot. 

 The spiral part of the operculum is often worn away. They are repre- 

 sented in the Himalayan regions by Paludomus; which, with the 

 fringed mantle of Melania, has the annular operculum of Paludina. 

 In the West Indian islands and the tropical districts of South America 

 are found a group of shells differing from the typical Melanias in 

 having the pillar sharply notched ; they are called Hemisinus. The 

 genus Melanopsis, which is peculiar to the old world, being found from 

 Spain to New Zealand, consists of stumpy shells notched for the siphonal 

 fold, and furnished with a lump at the suture like Bullia and Polinices. 

 The elongated forms, found in Africa and the tropical East Indian 

 islands, are called Pirena, and have the lip very much produced in 

 front. The shell of Clionella has a distinct notch in the outer lip like 

 Drillia. It inhabits the African rivers, but the animal has not yet 

 been examined. Lastly, in the Southern States of America are found 

 the beautiful shells of Io, in which there is not merely a notch, but a 

 distinct, straight canal, to convey water to the gill cavity. 



Family Cerithiad^;. (Cerites.) 



The Cerites are a very numerous tribe of turreted shells, with a notch 

 or canal at the bottom of the pillar, in consequence of which they were 

 classed with the Muricids by Lamarck. The animals however closely 

 resemble the Periwinkles, Melanias, &c. They are known from the 

 latter by the absence of fringe on the mantle, by their strongly sculp- 

 tured shells, and by the greater development of the siphonal fold in 

 the mantle. This is never produced into a projecting recurved pipe, 

 as in the notched Proboscidifers. The Cerites are found in all parts of 

 the world; but the typical species do not ascend higher than the 

 Mediterranean. Some of the species emit a bright green fluid when 

 disturbed. Like their neighbors the Periwinkles, they are extremely 

 plentiful in individuals. They inhabit the ebb-tide line and deeper 

 waters round shores, and certain groups are very plentiful in brackish, 

 water and salt marshes. The shells of Cerithium have a very short, 

 slightly bent canal, and an operculum like Litorina, of few whirls. 

 Rhinodavis has the canal bent back like Cassidaria, with a fold on 

 the pillar, and a porcellanous texture in the shell. The fossil group 

 Nerincea, found in the older secondary rocks, is like an exaggerated 

 Bhinodavis, with a large number of plaits, both on the pillar and in- 

 side the whirls. The shell is often very slender like Terebra, which 

 it may have resembled in habits. One species of Rhinodavis has been 

 figured by Adams with a muricoid operculum, but other species are 

 known to possess the paucispiral form. In the remaining members of 

 the family, the operculum is round, with many whirls. The dwarf 

 Cerites of the northern seas have only a slight pillar notch, and bear 

 some resemblance to the elongated Rissoas; they are called Bittium. 



The fresh water Potamides are known by their brown epidermis, 

 and lip produced in front. The fossil forms are very numerous and 

 beautiful in the tertiary strata. In Pyrazus the outer lip is arched 



