36 MANUAL OF THE MOLLTJSCA. 



kind ; but the attached valve of chama has its umlo tamed 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. 



The 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 

 undulata^ Fig. 89 ). Occasionally the inner strata of porcel- 

 lanous shells are differently coloured from the exterior, and the 

 makers of shell -cameos avail themselves of this difference 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 pleuronedes) have the upper valve richly tinted, whilst 

 the lower one is colourless. The backs of most spiral shells 

 are darker than the under sides ; but in ianthina the base of 

 the shell is habitually turned upwards, and is deeply dyed with 

 violet. Some colours are more permanent than others ; the red 

 spots on the naticas and nerites are commonly preserved in 

 tertiary and oolitic fossils, and even in one example (of n. sub- 

 costata, Schl.) from Devonian limestone. Terehratula hastata^ 



and some pectens of the car- 



jg^ boniferous period, retain their 



^jl^^^B markings ; the orthoceras angu- 



"^Ns. /ii^«f^^^[?^Jp| liferus of the Devonian beds has 



^**'^'^PC'!*ft'^I^B 2ig-zag bands of colour ; and a 



^^;^5^^M^^^^^ terehratula of the same age, 



^^T^^^^^^^^^^ from Arctic North America, is 



* ^ r:^^^^:^^ ornamented with several rows 



of dark red spots. 

 Vig,28. li'ochus.iziphinusA The operculum. Most spiral 



shells have an operculum, or lid, with which to close the aper- 

 ture when they withdraw for shelter {See Qasteropoda). It is 

 developed on a particular lobe at the posterior part of the foot, 



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

 on an orange ground ; on c. tuberosa, and madagascariensis, white upon dark claret- 

 colour ; on c. rufa, pale salmon-colour on orange ; and on strombus gigas, j'^ellow on 

 pink. B}^ filing some of the olives (e.g. oliva utriculus) they may be made into very 

 di^rent-coloured shells. 



t Trochus ziziphinus, from the original, taken in Pegwell Baj' abundantly. This 

 species exhibits small tentacular processes, neck-lappets, side-lappets, tentacular 

 filaments, and an operculigerous lobe. 



