2 34 Shell Life 



possessed by some of the Crabs, and are therefore 

 supposed to be able to keep a sharp look-out as a 

 sentinel should. These eyestalks are of the same 

 length as the tentacles, to which they are united. 

 The shell is very like that of Hydrobia. The Dun 

 Sentinel {A. grayana) lives in brackish water, coming 

 up the Thames as far as Greenwich. Another 

 of these small-sized mollusks is the Sl'enea 

 plaiiorhis, whose reddish or tawny shell at 

 lirsb siofht looks like one of the small fresh- 

 water Flat-coiled Shells (Planorhis), whence 

 its specific name. The spire is flattened, the four 

 rounded whorls loosely coiled, the mouth round, 

 and the spiral operculum horny. In habit it is very 

 like the Winkles, for though it has been dredged 

 from a depth of 40 fathoms, and may be taken from 

 Confervce in the rock-pools, it also feeds upon Lichina 

 pygmcea, which spends half the day submerged and 

 the other half dried and shrivelled in the air. A 

 somewhat similar but less depressed shell is the 

 White Belted-shell (Adeorhis suhcarinatus), which 

 makes up for small dimensions by having a 

 long scientific name, as many other small 

 creatures do. There are only 4 whorls, and 

 these have 6 strong narrow ridges or keels. 

 The large mouth does not show quite so even 

 an outline as in our figure, the ridges usually 

 producing corresponding indentations of the mouth. 

 The operculum is thin. There is a large umbilicus. 

 It is a local species, occurring at various places on 

 the south and west coasts, and at a few places in 

 Scotland and Ireland. 



The obvious natural relationships of the species 



