MARINE MOLLUSCS 243 



an imperfect condition. In the Atlantic and the Mediterranean it 

 sometimes abounds in such multitudes as to distinctly colour the 

 surface of the sea. 



It will be seen that the shell is round, with a well-formed spiral. 

 The spire is white, but the base is of a deep violet colour. The 

 animal is very remarkable in some respects. In the first place, 

 though it has pedicels similar to those on which the eyes of the 

 higher univalves are placed, yet it has no eyes. Then the foot, 

 which is in itself small, secretes a float or raft so large that it 

 cannot be retracted into the shell, with numerous air vesicles to 

 render it light, and the egg-capsules of the animal are attached to 

 the underside of this. The animal has no power of sinking, but 

 lives exclusively at the surface ; and, when disturbed, it exudes 

 a violet fluid that colours the surrounding water. It is apparently 

 the only gasteropod that lives in the open sea and has a large and 

 well-formed spiral shell. 



Passing now to the family TurbinidcB we meet with turbinated or 

 pyramidal shells that are of a brilliant pearly lustre within, and 

 frequently without also when the epidermis is removed. The 

 animals inhabiting them have well-formed heads with a short 

 muzzle, long and slender tentacles, and eyes mounted on peduncles. 

 The sides are ornamented with fringed lobes and several tentacle - 

 like filaments, and the aperture of the shell is closed, when the 

 animal is retracted, by a spiral operculum. They are all vegetable 

 feeders ; and, as is usual with the plant-eating molluscs, the teeth 

 on the lateral portions of the lingual ribbon are very numerous. 



We have a few common species belonging to this group, mostly 

 members of the typical genus Troclius and commonly known as 

 Top Shells. In these the shell is a pyramid formed of numerous 

 flat whorls, with an oblique and rhomboidal aperture. Of the three 

 species figured (including two on Plate V.) T. umbilicatus and the 

 Large Top (T. magnus) are umbilicated, the umbilicus being very 

 large in the latter ; and the former is characterised by the zigzag 

 greyish or reddish markings that run radially across the whorls. 

 The other (T. zizyphinus) is usually of a yellowish or pink colour 

 and has no umbilicus. 



The same family contains the pretty little Pheasant Shell 

 (Phasianella pullas), which is richly coloured with red, brown, 

 and yellow on a light ground ; and Adeorbis subcarinatus, shown 

 in the same group. 



The well-known Periwinkle (Littorina littorea) and the species 



