46 MANUAL OF THE MOLLUSCA. 



discovered whole races of netitina, paludina, and melanopsis, wit; 

 whirls ribbed or keeled, as if through the unhealthy influence o 

 brackish water. The fossil periwinkles of the Norwich Crag ar 

 similarly distorted, probably by the access of fresh-water ; paralle 

 cases occur at the present day in the Baltic. 



Reversed shells. Left-handed, or reversed varieties of spira 

 shells have been met with in some of the very common species 

 like the whelk and garden-snail. Bulimus citrinus is as often 

 sinistral as dextral ; and a reversed variety of/usus antiqum wa 

 more common than the normal form in the pliocene sea. Othe 

 shells are constantly reversed, as pyrula perversa, many species o 

 pupa, and the entire genera, clausilia, cylindrella, physa, and tn 

 pJioris. Bivalves less distinctly exhibit variations of this kind 

 but the attached valve of chama has its umbo turned to the righ 

 or left indifferently ; and of two specimens of lucina childreni in 

 the British Museum, one has the right, the other the left valv 

 flat. 



The colours of shells are usually confined to the surface beneat 

 the epidermis, and are secreted by the border of the mantle 

 which often exhibits similar tints and patterns (e. g. voluta undu 

 lata, fig. 73). Occasionally the inner strata of porcellanous shell 

 are differently coloured from the exterior, and the makers of shell 

 cameos avail themselves of this difference to produce white o 

 rose-coloured figures on a dark ground.* 



The secretion of colour by the mantle depends greatly on th 

 action of light ; shallow- water shells are, as a class, warmer am 

 brighter coloured than those from deep water ; and bivalve 

 which are habitually fixed or stationary (like spondylus and pecten 

 pleuronectes) have the upper valve richly tinted, whilst the lowe 

 one is colourless. The backs of most spiral shells are darke 



* Cameos in the British Museum, carved on the shell of cassis cornuta 

 are white on an orange ground ; on c. tuberosa, and madagascariensis, whit 

 upon dark claret-colour ; on c. rufa, pale salmon-colour on orange ; and on 

 strombus gig as, yellow on pink. By filing some of the olives (e. g. oliva utri 

 culus] they may be made into very different coloured shells. 



