UNIVALVES. 59 



its left-handed shell; the amber-coloured Succinea, in which the 

 mouth is more than half the shell's height ; the richly variegated and 

 polished Phasiandla ; and Assiminea, with its inner lip thick and 

 outer lip thin. Seven genera are left. In two of these the shell is 

 thin Bithynia, which has a thick outer lip, and Hydrubia, which has 

 a thin outer lip. In three the shell is moderately thick ; one, 

 Cyclostoma, having a nearly circular mouth, the other two being 

 separable on size alone, Rissoa never exceeding half an inch, and 

 Viviparus, otherwise Paludina, never measuring less than an inch. 

 This reduces the seven to two in which the shell is particularly 

 solid ; and it is smooth in Barlee'm and heavy and striated in 

 Littorina, as everyone knows who has picked up a periwinkle. 



With that we end the British gastropods having entire mouths, 

 among whom are included all our land and fresh-water shells but 

 one. That one is Achatina acicula, which can be distinguished from 

 the rest of the notched group by its transparency. It is a diminutive 

 species, not much more than an eighth of an inch long, and seldom 

 found alive owing to its living underground among the roots of the 

 trees and plants. It has no eyes, and its lower tentacles are only 

 discoverable with difficulty as a pair of almost invisible knobs, and, 

 with its thin white shell, is altogether a most unexpected representa- 

 tive of a genus which contains over 400 species, mostly African, 

 including the largest of living land-snails, which is more than six 

 inches in length. 



Leaving the land for the sea, we can divide the non-trausparent 

 group into 



Shell convolute. 



Shell with a long narrow mouth. 

 Shell with a curved canal. 

 Shell with a straight canal. 



Those with a convolute shell are three in number, and in two of 

 them the spire is hidden. These are Ovula and Cyprcea, the cowry. 

 In the cowry the outer lip is ribbed, in Ovula it is plain. Although 

 we have thus brought them together, owing to their being convolute 

 and having hidden upper whorls, their shape is very different, as can 

 be seen in the illustrations ; and the cowry is so well known that it 

 would almost be sufficient to say that the pair consists of the cowry 

 and another, for there is only one species of each. In one other 

 convolute shell, Marginella, the spire is not turned in, but is very 

 low. It looks like a cowry that had thought better of it, and is a 

 singularly graceful little shell, bright and polished, and ribbed only 

 on the outer lip, the rib being very faint. As with the other two, 

 we have only one species of the genus, that being Icsvis. Briefly 

 then we have : 



Shell convolute 



Spire not hidden Marginella. 

 Spire hidden. 



Outer lip plain Ovula. 



Outer lip ribbed Cypraa. 



