STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 47 



than the under sides ; but in iantJiina 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. subcostata schl.) from 

 Devonian limestone. Terebratula Jiastata, and some pectens of 

 the carboniferous period, retain their markings; the ortlioceraa 

 anguliferus of the Devonian beds has zig-zag bands of colour ; 

 and a terebratula of the same age, from arctic North America,* 

 is ornamented \vith several rows of dark red spots. 



The operculum. Most spiral ^-f 



shells have an operculum, or lid, <?f^^-l4 



mth which to close the aperture 

 when they mthdraw for shelter 

 (see gasteropoda). It is deve- 

 loped on a particular lobe at the "^ — ^^ - -^^ji^^^^y;-^^^ 

 posterior part of the foot, and » 

 consists of horny layers, some *^^- ^^- ^'ochus ziziphinus:\ 

 times hardened ^nth shelly matter (fig, 28). 



It has been considered by Adanson, and more recently by 

 Mr, 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 bj/ssus of the 

 bivalves (Loven) ; and in function it is like the plug with which 

 unattached specimens of bijsso-arca close their aperture. {Forbes.) 



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

 that its modifications present few points for comparison; but 

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



* Presented to the British Museum by Sir John Richardson. 



t Trochus ziziphinus, from the original, taken in Pegwell Bay abundantly. 

 This species exhibits small tentacular processes, neck-lappets, sidc-lappets. 

 tentacular filaments, and an opcrculigcrous lobe. 



:|: Parts which correspond in their real natiu-e— (their origin and develop- 

 ment) — are termed homolorjous ; those which agree merely in appearance, or 

 office, are said to be analogous. 



