Molluscs. 27 



or whose empty shells lie on the beach, are conformed ver}' much 

 like the Whelk. The commoner forms, some of which are figured 

 here (Fig. 24), are easily known by the shape and sculpture of 

 the spiral shell. We may mention the Periwinkles (Littorina 

 liitorea and I. rudis) of commercial importance, the latter 

 of special interest for the long period, up to a month, it may 

 remain beyond the reach of the sea, and as a viviparous marine 

 mollusc ; the Sting-mnkle {Murex erinaceus) ; the Purple 

 (Purpura lapidus) ; the Dog whelk [Nassa reticulata) ; the Sea- 

 snail (Naiica catena) ; the Tower shell (Turriiella communis) ; 

 and the beautifuUy marked species of Trochus, such as Trochus 

 cinerarius and T. zizyphinns (Fig. 36), the latter, with its conical 

 smooth shell, resembling a top. Some of the M-urex and Purpura 

 secrete a peculiar fluid which gives a purple-coloured dye, a 

 member of the former genus being the shell-fish from which 

 the ancients obtained their well-known Tyrian purple. 



A very common Gastropod is the Limpet (Patella vidgata), 

 found between tide-marks on all rocks, to which it adheres 

 with extraordinary force by means of the circular foot. It is 

 further remarkable for the fact that the shell has acquired 

 a secondary s\anmetry ; that this symmetrj^ is not a primitive 

 feature is shown by the young, which has a coiled shell. Althougli 

 the same individual limpet may be seen day after day in exactly 

 the same position, the animal is not without powers of locomotion, 

 for, when covered by the tide, it often travels great distances 

 in search of sea weeds on which it feeds, invariably returning 

 to the same spot before the tide recedes. This " homing 

 instinct " is a most interesting trait in the habits of this curious 

 creature. 



The Ear-shell, or Ormer (FI allot is tnherculata), is a close 

 ally of the limpet, abundantly found on the coasts of the Channel 

 Islands and of Brittany. The much-flattened sheU has a low 

 spiral and is perforated by a series of holes, the object of which 

 is to allow the escape of the water aerating the gills in the mantle 

 cavity. The inner surface of the shell is beautifully coated 

 with mother-o'-pearl. Another type of shell of highly aberrant 

 appearance is the small Cowry (CyprcBa europoea), the reduced 

 British representative of the more ornamental exotic species 

 which adorn the shell-cabinet, and not unhke the Indo-Pacific 

 Money-cowry, which has long been the currency among the 

 negroes of some parts of Africa. The sliell of the Cowry, 

 with its oval shape and long, sUt-like opening, looks quite 

 symmetrical ; but when young it has the ordinary spiral 

 shape, and as it approaches the adult condition, a thick outer 



