SNAILS AND LIMPETS. 177 



* 



described as quoit-shaped, and the upper side is more conyex 

 than the lower: it is of a glossy reddish-brown coloxir, and 

 not quite Jin. in diameter. There are four whorls : the body- 

 whorl, which is larger than aU the rest, contains partitions 

 plainly visible through the shell. The spire is much depressed. 

 This snail lays about five eggs, and incloses them in an 

 orbicular capsule. The fry are hatched in from ten to four- 

 teen days. 



Planorbis niiidus is found in the sluggish and stagnant 

 water of nearly every part of Britain. It is very like the 

 P. lineatm, but is smaller, is without the internal partitions or 

 plates, and the spire of the shell is not so much depressed. 

 The animal is almost black, and is spotted with dark grey. 

 The tentacles are very slender and pointed at their tips. 

 The shell varies in both size and colour, but it is never, I 

 think, as much as Jin. in diameter. Its colour varies from 

 dark to light hom-colour, and is slightly prismatic. The 

 largest shells are generally the darkest. There are three or 

 four whorls. The body- whorl, which is very large in pro- 

 portion to the rest, partly covers the preceding whorl and 

 turns outward. The periphery is rather prominently keeled; 

 near the centre. The sutures are fairly deep; the aperture 

 is angular. The eggs of this snail, which are about four in 

 number, are inclosed in an orbicular capsule, and are hatched 

 in from ten to fourteen days. It is a very sluggish 

 mollusc, its shell sometimes being found covered with insect 

 egg-cases or with confervse. 



The following Planorbes are, as a rule, too small for the 

 ordinary aquarium, but they will be interesting, harmless, 

 and useful in those tanks in which fish are not kept : 



Planorbis contortus is a little snail found on plants in 

 clear, stagnant water throughout Britain, but it is rather local. 

 It is inactive. Its body is almost black and is tinged with 

 red. Its tentacles are unusually slender and of a brownish 

 colour. The shell is about ^in. in diameter, and of a brownish 

 hom-colour; it is flat above with a concavity in the middle, 

 and very concave below. There are eight very compact 

 whorls. The sutures are deep, and the aperture is narrow and 



N 



