THE SHELL. 25 



has its umbo turned to the right or left indifferently ; and of two 

 specimens of Lucina Ghildreni in the British Museum, one has 

 the right, the other the left valve flat. 



Colors of Shells. These 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 iindulata, liii, 14). Occasionally the inner strata of 

 porcellanous shells are differently colored from the exterior, and 

 the makers of shell-cameos avail themselves of this difference to 

 produce white or rose-colored figures on a dark ground. Cameos 

 carved on the shell of Cassis cornuta, are white on an orange 

 ground; on G. tuberosa, a,nd Madagascariensis (Ixii, 22), white 

 upon dark claret-color ; on C. rufa^ pale salmon-color on orange ; 

 and on Stronibus gigas (lix, 51), yellow on pink. By filing some 

 of the olives ( e. g. Oliva utriculus) they may be made into very 

 different colored shells. 



The secretion of color by the mantle depends greatly on the 

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

 brighter-colored 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 colorless. The backs of most spiral shells are darker 

 than the under sides ; but in lanthina the base of the shell is 

 habitually turned upwards, and is deeply dyed with violet. Some 

 colors 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 -A^. subcostata, Schl.) 

 from Devonian limestone. Tei-ebratida hastata^ and some Pectens 

 of the Carboniferous period, retain their markings ; the Ortho- 

 ceras anguliferus of the Devonian beds has zigzag bands of 

 color ; and a 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 aperture when they withdraw for shelter 

 (xliii, 1). It is developed on a particular lobe at the posterior 

 part of the foot, and consists of horny layers, sometimes 

 hardened with shelly matter. 



It has been considered by Adanson, and more recently by Dr. 

 Gray, as the equivalent of the dextral valve of the conchifera ; 

 but however similar in appearance, its anatomical relations are 

 altogether different. In position it represents the byssus of the 

 bivalves ; and in function it is like the plug with which unat- 

 tached specimens of Byssoarca close their aperture. 



Homologies of the Shell. The shell is so simple a structure 

 that its modifications present few points for comparison ; but 

 even these are not whollj^ understood, or free from doubt. The 

 bivalve shell may be compared to the outer tunic of the ascidian, 



