46 MANUAL OF THE MOLLUSCA. 



discovered whole races of neritina, paludina, and melanopsis, with j 

 Avliirls ribbed or keeled, as if through the unhealthy influence of \ 

 brackish water. The fossil periwinkles of the Nor^^dch Crag are ; 

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

 cases occur at the present day in the Baltic. ; 



Reversed shells. Left-handed, or reversed varieties of spiral j 

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

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

 sinistral as dextral ; and a reversed variety oifusus antiquus wasi 

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

 shells are constantly reversed, as pyrula perversa, many species oq| 

 pupa, and the entke genera, clausilia, cylindrella, phjsa, and tri*n 

 pJioris. Bivalves less distinctly exhibit variations of this kind ; 4 

 but the attached valve of cTcania has its umbo turned to the right J 

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

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

 flat. 



TJie colours of shells are usually confined to the surface beneath 

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

 which often exhibits similar tints and patterns {e. g. valuta undu- 

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

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

 cameos avail themselves of this difference to produce white or' 

 rose-colom'ed figures on a dark ground.* 



The secretion of colour by the mantle depends greatly on the, 

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

 brighter coloured than those from deep water ; and bivalves"! 

 which are habitually fixed or stationary (like spondylus and pecten } 

 fleuronectes) have the upper valve richly tinted, whilst the lower f 

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



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

 are white on an orange ground ; on c. tiiberosa, and madagascarlensis, white \ 

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

 stromhus (jlgas, yellow on pink. By filing some of the olives {e. g. oliva utri- 

 cuius) they may be made into very different coloured shells. 



