8& MANUAL OF THE MOLLUSGCA. 
kind; but the attached valve of chama has its uwmbo 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. 
The colowrs 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. voluta 
undulata, Fig. 89). Occasionally the inner strata of porcel- 
lanous shells are differently coloured from the exterior, and the 
makers of shell-cameos ayail 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 pleuronectes) 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 ¢anthina 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. Terebratula hastata, 
and some pectens of the car- 
boniferous period, retain their 
markings; the orthoceras angu- 
liferus of the Devonian beds has 
zig-zag bands of colour; anda 
terebratula of the same age, 
from 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 GASTEROPODA). It is 
developed en a particular lobe at the posterior part of the foot, 
Fig. 28. Trochus zziphinus.t 
* Cameos, in the British Museum, carved on the shell of cassis cornuta, are white 
on an orange ground ; onc. tuberosa, and madagascariensis, white upon dark claret- 
colour; on ¢. rufa, pale salmon-colour on orange; and on strombus gigas, yellow on 
pink. By filing some of the olives (e.¢. oliva utriculus) they may be made into very 
different-coloured shells. 
¢ Trochus ziziphinus, from the original, taken in Pegwell Bay abundantly. This 
species exhibits small tentacular processes, neck-lappets, side-lappets, tentacular 
filaments, and an operculigerous lobe. 
