208 LECTURES ON MOLLUSCA. 



and twisted over the canal, making it somewhat tubular. Lampania 

 has a shell shaped like Pirena. Terebralia has a broad pyramidal 

 shell with flattened whirls. The mouth is square, with a deeply 

 waved outer lip, and a plait on the twisted pillar. The T. telescopium 

 is so plentiful near Calcutta as to be burnt for lime. In the very 

 pretty group CeritMdea, the notch is almost obsolete; the mouth is 

 round; and on reaching maturity it is reflected back. The shells are 

 very thin and light, and very commonly decollated at the point. The 

 animals live in mangrove swamps, estuaries, and salt marshes. They 

 crawl so much out of the water that they have been taken for land 

 shells ; and in the dry season, they hang themselves from the man- 

 groves by glutinous threads. 



It is not known whether the animals of Triforis are most related to 

 Cerithium or Cerithiopsis . Perhaps among the lefthanded species 

 which have been grouped together under that name, there may be 

 found some of each kind. (See CeritMopsidce, above, page 185.) The 

 ancient Cerites are of the Nerincea form : the typical race does not 

 appear till the cretaceous age, but rapidly develop in the tertiaries. 



Family Turritellid^e. (Screiv-Shells.) 



The Screws are to the vegetarian section of Comb-gilled Crawlers, 

 what the Augers are to the boring tribe. The shell is very long, and 

 regularly pointed; the whirls ven r numerous and generally rounded; 

 and the texture for the most part strong, and somewhat porcellanous. 

 The creatures do not drop away the pointed end, like Cerithidea and 

 Truncatella ; but they are fond of marking off the left portions, one 

 after another, by plain partitions. In external appearance the Screw- 

 mollusks are extremely like the Melanias and Cerites. They have a 

 very short foot, squared in front; and a short, thick muzzle, somewhat 

 united to the foot below. The mantle is fringed even more prettily 

 than in Melania. The operculum is round, with many whirls, as in 

 Potamis; often with a thin fringe at the edge. As the foot is grooved 

 below, the creature has the power of moving right and left altern- 

 ately. But the heavy, long spire and short foot betokens in gen- 

 eral a sluggish habit ; and the Screws generally repose in stiff mud 

 like the Augurs, in rather deep water. But while the blind Augurs 

 grub in the mud for their prey, the Screws expose their delicate 

 fringe and long thin tentacles with eyes on stumps beneath to search 

 for their food above the surface. The teeth are broad and extremely 

 finely serrulated, like those of Paludina; the tongue-ribbon being 

 very small. There is a rudimentary breathing fold, but the pillar 

 is not notched. The gill-comb is extremely long. 



The animals have not been examined in a sufficiently large number 

 of species to ascertain whether there are any generic differences among 

 them. They have been thus separated provisionally, according to the 

 shell. Turritella has the mouth round. In Haustator, it is somewhat 

 squared by the shouldering of the base : very fine species of this group are 

 found in west tropical America. In Torcida, the middle of each whirl 

 is curiously hollowed out. The shells of Mesalia are short, with flat- 



