GASTROPODS. 347 



may be from about twenty to forty clays before the young snail eats through the 

 skin or shell of the egg, and this it is said to entirely consume. Although H. 

 pomatia is commonly styled the edible snail, it should be remembered that other 

 species are also considered a delicacy ; H. aspersa, H. naticoides, and H. 

 vermicidata being commonly eaten in parts of Italy and Sicily ; while in Naples, 

 II. ligata and H. lucorum are also to be seen for sale. H. mazzullii, which is 

 collected near Palermo, possesses the power of boring into the rock ; this being 

 probably effected by means of the radula. The supposed object of these excava- 

 tions is to secure a place of refuge during the period of excessive heat, and also to 

 obtain a supply of lime for the formation of the shell. The Philippine Islands are 

 the home of a beautiful genus of snails (Helicostyla), many of which are clothed 

 with a peculiar periostracum, which, when wetted, becomes more or less transparent, 

 disclosing the colour of the shell beneath. Two or three of the species are of a 

 bluish tint, a colour not found among other land-shells. 



The important genus Bidimiis (now abolished in favour of Strophocheihis) 

 formerly contained a large number of species from all parts of the world, but is 

 now limited to a comparatively few forms occurring in South America ; S. oblongus 

 being one of the commonest and best known species. Another, S. maximus, is 

 sometimes as much as 6 inches in length. The majority of the other species, 

 originally classed in Bidimus, are now located in Bidimidiis and its sections, or in 

 Bidiminus. These genera are distinguished by differences of the jaw and radula. 



There are several other families of land-shells, which can only 

 Other Families. . . 



be briefly referred to. The Cylindrellidw are very elongate, many- 



whorled shells, which occur principally in the West Indies and South America ; 



many of them being remarkable for their slender forms and exquisite sculpture. A 



peculiarity of Cylindrella consists in the shell being almost invariably truncated, 



that is, the upper whorls are broken off when the animal reaches maturity. This 



may be done to lighten the shell, which otherwise would be too long for the 



mollusc to carry erect. The Pupidcs comprises a number of forms which are 



conchologically very dissimilar. The typical Pupa includes minute creatures, and 



appears to be found in all parts of the globe, but especially in temperate and 



mountainous regions. They are gregarious and live in moss, under stones, in the 



crevices of old walls, or among the roots of grass and other plants. Most of the 



species have what are called " teeth " within the aperture of the shell, and these in 



some species are so numerous as to almost close the opening, and 



thus make it, one would suppose, a matter of some difficulty for the 



animal to squeeze through them when emerging from its shell. 



Vertigo is a genus similar to Pupa ;is regards the shell, but separable 



on account of the animal having only one pair of tentacles, the lower 



pair being entirely wanting. The shells, according to the species, 



are dextral or sinistral. Clausilia is remarkable for the large 



number of species, the general similarity in the form of the slender 



shell, and the peculiar process within it, which serves as a door to 



shut in the animal when retracted. The shells are almost invariably reversed, and 



furnished with two or three folds or plica? within the mouth, and other lamella? 



still further within, which can only be detected by the transparency of the shell 



