3 86 



MOLLUSCS. 



size; Cerithium giganteum, an Eocene form, attaining quite a foot and a half 

 in length. 



One of the most curious groups of gastropods is that of the worm-shells 

 ( Vermetidce), in which the shells might be mistaken for the tubes of marine 

 worms, as they are similarly twisted and attached to stones and other substances 

 in the same way. They are free and spiral in early life, and crawl about like 

 ordinary gastropods, but they afterwards settle down and become stationary for 

 the rest of their existence. In these circumstances, a walking-foot, being of no 

 further use, becomes modified into a mere support of the operculum. The animals 

 are worm-like, with a short proboscis, horny jaws, and radula, and the head 

 supporting two short tentacles, with the eyes at the base. The species are not 

 numerous, and occur chiefly in warm and temperate seas. The members of the 

 extensive family of the Melaniidce are inhabitants of fresh water, and are 

 abundant in all subtropical parts of the globe. The shells are not, as a rule, 

 attractive, being clothed with a dark or olivaceous periostracum. Some are long, 

 slender, and acute, others quite globular. Perhaps the most remarkable form is 

 Tiphobia horei, an inhabitant of Lake Tanganyika, in Central Africa. In the 

 typical Melania the aperture of the shell is entire, but in some of the other genera, 

 such as Melanopsis and Fawnus, it is distinctly notched in front. The animal is 

 provided with a horny operculum, and many are viviparous. Hundreds of species 

 have been described, but many, as is the case in all fresh- water groups, are 

 distinguished by very slight differences. The Strepomatidw, or Pleuroceridce, are 

 the North American representatives of the Melaniidce of the Eastern Hemisphere, 

 from which they are distinguished by the absence of the marginal mantle-fringes, 

 and in being oviparous in their mode of reproduction. In certain places they 

 abound in such countless numbers as almost to cover the bed of some of the 

 streams in Tennessee and Alabama. About five hundred forms have been recognised. 

 The genus lo contains the largest and most striking species of all. They are 

 short, spindle-shaped shells, often with nodose or spinose whorls, and with the 

 aperture prolonged into a distinct anterior canal. They are restricted to certain 

 parts of Virginia and Tennessee. The little sea-snails known as periwinkles 

 (Littorinidce) are dwellers on the shore. They are all vegetarians, and occur in 



the Arctic and Antarctic regions, as well as in 

 temperate and tropical shores, wherever they 

 can find rocks and stones to crawl upon. Some 

 occur at low water ; others live at high-water 

 mark, or where they are only occasionally 

 reached by spring tides. Some ascend the 

 mangrove trees, and have been found hundreds 

 of yards from the sea. Four species of 

 Littorina inhabit the English coast, the 

 commonest being the well-known L. littorea, 

 which is consumed in such enormous quan- 

 tities. The periwinkles have horny jaws, and 

 a very long radula, sometimes two or three times as long as the animal itself ; and 

 they are all furnished with a horny operculum to protect themselves with, when 



spawn of periwinkle (magnified). 



