TROCHOID^E ; PERIWINKLE; PALUDINA. 379 



there did not appear to be any real shell at all ; the animal being 

 inclosed in cases made up of little stones and grains of sand, 

 agglutinated together with such exactness, as to form the fac- 

 simile of a real Trochoid shell. The Trochus longispina from 

 India has the circumference ornamented with a row of long 

 spines, of a silvery or gold colour, placed at regular distances. 

 In the Turbo, to which the common Periwinkle belongs,* the 

 mouth of the shell is quite round ; and it is in this genus that we 

 find the most massive and stony opercula. The Periwinkle is 

 very generally used as an article of food in the neighbourhoods 

 in which it abounds. It is considered in Sweden to afford a 

 sign of the coming weather ; the peasants having observed that, 

 whenever the Periwinkles ascend the rocks, it is a sure sign of a 

 storm being near, their instinct having taught them to place them- 

 selves out of the reach of the dashing of the waves ; when, on the 

 contrary, they make a descent upon the sand, it is an indication of 

 a calm. The number of known recent species is between thirty 

 and forty; and, as in the former case, those of temperate climates 

 are much surpassed in size by the inhabitants of tropical seas. 

 Allied to the Turbo is the Scalaria, in which the turns of the 

 spire come in contact with each other only by their ribs, and of 

 which the principal species the Scalaria preciosa, or Wentle- 

 trap was long famous on account of the high price given for it 

 by shell-collectors. 



920. The preceding genera are all inhabitants of salt water 

 only ; and their shells, as well as their bodies, have a very cha- 

 racteristic and peculiar structure. We have now to advert to 

 some, in which there is a considerable resemblance, in one or 

 both of these particulars, to the Helices and their allies. First 

 among them may be mentioned the Paludina^ which nearly ap- 

 proaches the Turbo in the form of its shell, but is an inhabitant of 

 fresh water ; it possesses, however, the rudiments of a siphon, for 

 the introduction of water into the respiratory cavity. In the com- 

 mon species, Paludina mvipara (Fig. 576), the young are produced 

 alive, the eggs being hatched within the oviduct. The lanthina, 

 or Violet Snail, is a beautiful little Mollusk, having a delicate 



* This is referred by most Naturalists to a new genus, Littorina. 



