STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 47 



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. subcostata schl.) from 

 Devonian limestone. Terebratula hast at a, and some pectens of 

 the carboniferous period, retain their markings ; the orthoceras 

 anguliferm of the Devonian beds has, zig-zag bands of colour ; 

 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 

 (see gasteropoda). It is deve- 

 loped on a particular lobe at the 



posterior part of the foot, and 



, PI i Fig, 28. Trochus ziziphinus.^ 



consists of horny layers, some 



times hardened with 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 byssus of the 

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

 unattached specimens of bysso-arca close their aperture. (Forbes.) 



Homologies of the shell. J 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 tlie British Museum by Sir John Richardson. 



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

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

 ;entacular filaments, and an operculigerous lobe. 



Parts which correspond in their real nature (their origin and develop- 

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

 )ffice, are said to be analogous. 



