46 MANUAL OF THE MOLLUSC A. 



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

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

 brackish water. The fossil periwinkles of the Noi'wdch 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 

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

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

 sinistral as dextral ; and a reversed variety of fusus antiquua was 

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

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

 pupa, and the entire genera, clausilia, cyUndrella, physa, and trU 

 phoris. Bivalves less distinctly exhibit variations of this kind ; 

 but the attached valve of cliama has its umbo turned to the right 

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

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

 flat. 



Tlie 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. roluta undu- 

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

 are difl'erently coloured from the exterior, and the makers of shell- 

 cameos avail themselves of this difi"erence to produce white or 

 rose-coloured 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 and 

 brighter coloured than those from deep water ; and bivalves 

 which are habitually fixed or stationary (like spondylus and pecten 

 pleuronectes) have the upper valve richly tinted, wliilst the lower 

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



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

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

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

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

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



