1 88 MOLLUSCA. 



are not known in the fossil state. The Lamp-shells and their 

 allies (Brachiopoda) possess a bivalve shell, consisting of two 

 pieces or " valves," which are more or less highly calcareous. 

 Coming to the higher Mollusca, the true bivalve Shell-fish 

 (Lamellibranchiata), as their common name implies, have also 

 a bivalve shell ; but this is distinguished from the shell of the 

 Brachiopods by sufficiently good characters. No Lamelli- 

 branch is destitute of a shell, and the remains of this class 

 occur more or less abundantly in all deposits except the most 

 ancient. The ordinary univalve Shell-fish (Gasteropoda), as 

 indicated by their common name, have usually a shell com- 

 posed of a single piece or "valve." In many Gasteropods, 

 however, there is either no shell at all, when the animal is said 

 to be " naked " (as in the Sea-slugs), or the shell is quite rudi- 

 mentary, and is concealed within the mantle (as in the ordin- 

 ary slugs). In other Gasteropods again (viz. in the Chitons), the 

 shell is " multivalve," consisting of eight pieces or valves placed 

 one behind the other. Most, however, of the " multivalve " 

 shells of older writers are really referable to the Cirripedia. 

 In the minute Oceanic Molluscs which form the class Ptero- 

 poda, the animal is sometimes naked, but is more usually 

 protected by a symmetrical glassy shell, which is always uni- 

 valve. In the class of the Cephalopoda, finally, great diversity 

 exists in the character of the skeleton. All the ordinary Cut- 

 tle-fishes have an internal skeleton, embedded in the mantle, 

 and not visible externally. This internal skeleton may be 

 calcareous or horny, and it may be of a very complicated 

 nature ; but it merely serves to support the soft parts of the 

 animal, and it does not form an external case in which the 

 animal lives. In one Cuttle-fish only (viz., the Argonaut or 

 Paper Nautilus), is there an external shell, but the nature of 

 this is quite peculiar, and it cannot be compared with the 

 shell of any of the ordinary Molluscs. In another group of 

 the Cephalopoda, represented at the present day by the Pearly 

 Nautilus, there is a well-developed external shell, which is 

 always composed of a single piece, and is always chambered, 

 the animal living in the last and largest chamber of the shell. 



In composition the shell of the higher Mollusca consists of 

 carbonate of lime usually having the atomic arrangement of 

 calcite with a small proportion of animal matter. In the 

 Pholadida, however, the calcareous matter exists in the allo- 

 tropic condition of arragonite, which is very much harder than 

 calcite. As regards their texture, three principal varieties of 

 shells may be distinguished viz., the " porcellanous," the 

 " nacreous," and the " fibrous." In the nacreous or pearly 



