412 MOLLUSC A. 



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 Pholadidce, however, the calcareous matter exists in the 

 allotropic condition of arragonite, which is very much harder 

 than calcite. As regards their texture, three principal varie- 

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

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

 shells, as seen in " mother-of-pearl," the shell has a peculiar 

 lustre, due to the minute undulations of the edges of alter- 

 nate layers of carbonate of lime and membrane. The " fib- 

 rous " shells are composed of successive layers of prismatic 

 cells. The " porcellanous " shell has a more complicated 

 structure, and is composed of three layers or strata, each of 

 which is made up of very numerous plates, " like cards 

 placed on edge." The direction in which these vertical plates 

 are placed is sometimes transverse in the central layer, and 

 lengthwise in the two others ; or longitudinal in the middle, 

 and transverse in the outer and inner strata. 



From their so commonly possessing hard structures, 

 whether external or internal, no fossils are more abundant 

 or important than Molluscs. As regards the general dis- 

 tribution of the Mollusca in time, the sub -kingdom com- 

 mences its existence in the Cambrian period, in which the 

 classes of the Polyzoa, Brachiopoda, Lamellibranchiata, Ptero- 

 poda, Gasteropoda, and Cephalopoda are more or less certainly 

 represented. 1 Speaking generally, the chief representatives 

 of the Mollusca in Palaeozoic time are the chambered Cephal- 

 opods (Tetrabranchiata) and the Brachiopoda ; in Mesozoic 



1 The Brachiopoda and Pteropoda have now been detected in the Lower 

 Cambrian. The Gasteropoda and Lamellibranchs are found in the Upper 

 Cambrian. The Cephalopoda do not make their appearance till near the sum- 

 mit of the formation, and are then poorly represented (so far as known, by 

 Orthoceras only). Lastly, the Polyzoa are of doubtful occurrence even in the 

 Upper Cambrian, unless we place the genus Dictyonema in this class. 



