38 



MAIiUAL OP IHE MOLLUSCA. 



kind ; but tlie attaclied valve of chama has its umho turned to 

 the right or left indifferently ; and of t-^o specimens of lucina 

 cMldreni in the British Museum, one has the right, the other 

 the left valve flat. 



The colours of sJteJIs 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. volida 

 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 dei^ends 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 

 Ijeden pleuronectes) have the upj^er 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 examj^le (of n. sub- 

 cosiata, Schl.) from Devonian limestone. Terehrahda hastata, 



and some pectens of the car- 

 boniferous iDcriod, retain their 

 markings; the ortlwceras angu- 

 h' ferns of the Devonian beds has 

 zig-zag bands of colour ; and a 

 tcrchratida of the same age, 

 fi'om Arctic North America, is 

 ornamented with several rows 

 of dark red spots. 



The operculum. Most spiral 

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

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

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



Fig. 23. Trochus ziziphimis. t 



* Cameos, in the British Museum, cai-ved on the shell of cassis cormda, are white 

 on an orange ground ; on c. iuherosa, and inadagascariensis, ■wliite upon dai'k claret- 

 colour ; on c. ?•!//«, pale salmon- colour on orange; and on strombus gigas, yellow on 

 pink. Bj' filing some of the olives (e.g. oliva utriculus) they may he made into very 

 different-coloured shells. 



t Trochus zizij)hi7ius, from the original, taken in Pcgwell Bay abundantly. This 

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

 filament*, and an operculigerous lobe. 



