THE YOUNG NATURALIST. 



881 



attached to the hinder part of the animal in 

 such a position that when it retreats within 

 its shell the operculum closes the aperture ; j 

 as the shell increases in size, and the aperture 

 becomes larger, the operculum also is built 

 up bv secretions from the mantle, the 

 deposit being made either spirally or in 

 successive rings, its structure and mode of 

 growth forming more or less distinctive 

 features of the species to which it belongs. 

 F AM I L Y , CYCLOSTO M AC E A 

 Genus, Cyclostoma. 

 There is only one British species to 

 represent this genus, the shell is spiral, 

 rather solid, and oval, the head of the animal 

 is lengthened out in front into a sort of snout, 

 it has slender tentacles, and the foot is 

 divided lengthwise, the two sides moving 

 alternately when the animal is walking, the 

 eyes are at the base of the tentacles and at 

 their outer side. The species is named 

 Eltgans. 



Elegans.— The shell is oval with raised 

 spire, whorls rounded, and sutures very 

 distinctly marked, it is spirally striated, the 

 aperture is nearh circular, the color is 

 mostly yellowish brown, sometimes with 

 dark j r spots; the operculum is fiat and spiral. 

 It is found in the South and Midland [.arts of 

 England, about the roots of shrubs, under 

 stones, &c. 



Genus Acme. 



This is the second genus of air-breathing 

 operculated Molluses, and like the preceding 

 one has only one species ; the shell is narrow 

 and cylindrical, the animal has long slender 

 tentacles, at the outer bases of which are 

 the eyes, the head is long and snout-like. 

 The British species is Li neat a. 



Lineata,.— The shell is very minute, only 

 about a twelth of an inch long, narrow and 

 cylindrical, with blunt apex ; it is transparent 

 and yellowish, the aperture is very small. 

 The operculum is horny and spiral. It may 

 be found throughout the country, under 

 stones and among moss, in damp situations. 



FAMILY, LITTORACE.A. 



Genus, Assiminea. 



This is the first genus of the operculated 

 water-breathing Molluscs ; the shell is spiral, 

 ra her solid and conical. The animal has 

 sh *rt stout tentacles, with the eyes at their 

 extremities, the head has a protruding muzzle, 

 in< entcd at the front, the foot is broad. 

 The species is named Grauana. 



Gray an a. — This is a small shell scarcely 

 a quarter of an inch in length, of a dull red- 

 dish brown colour ; it is conical with a pointed 

 spire, the aperture is rather small. It is the 

 on')- fresh-water representative Of the 

 Periwinkle family, which are all shore 

 molluscs, dwelling about the margin of the 

 water ; this one makes its home about the 

 mud and reeds by the river side where the 

 water is brackish, jt is found abundantly on 

 th - banks of the Thames about Greenwich 

 an 1 Woolwich. 



FAMILY, PERISTOMATA. 



Genus, Bythinia. 



The shells of this genus are conical, with 

 rounded whorls covered with a thin epidermis, 

 the lip is continuous round the aperture. 

 The Animals have long thread-like tentacles, 

 wi.h their eyes on swellings at their base on 

 the outer sides, they have a rather long 

 rounded proboscis notched at the front. 

 The opsrcula vary in their mode of growth 

 in the different species. The species are 

 Similis, TenlacuLit.i and Leachii. 



Similis. — This is a smaller shell than 

 A Gray-ana., similar to it in shape but with 

 the spire shorter, and with the sutures very 

 distinctly grooved. The operculum is spiral 

 of about two whorls only, with the nucleus 

 towards the side. It is found aboui Green- 

 wich in muddy ditches by the river side. 



rentacuiata. — The largest of the genus, 

 the shell attaining at full growth a length of 

 at out half an inch, it is oval, with conical spire, 

 whorls rounded, and aperture oval, its colour is 

 yellowish tinged with green. The operculum 

 is formed of concentrie rings. It may be 



