1899] Foord. — Carboniferous Brachiopoda and Mollusca. 71 



crinoids or sea-lilies, as well as the hard parts of corals and 

 foraminifera, played a most important part. It was this sedi- 

 ment, ultimately raised above the sea and gradually con- 

 solidated and altered, partly by the pressure of its own weight 

 and partly by chemical and other agencies, that was trans- 

 formed into the hard and compact limestone as we now see it 

 in quarries or other rock-cuttings. 



Fossils have been compared with the covers of books that 

 have had their leaves torn out, but, as with books, there is 

 generally sufficient in the title on the cover to guide us as to 

 their contents ; so with fossils, the shell, though devoid of its 

 constructor, often contains marks by which we may trace its 

 life-history, and it is here that zoology and palaeontology go 

 hand in hand ; we compare the extinct with the existing type, 

 and we are often surprised to find how little they differ from 

 each other. Thus, if a Lingula from the Silurian rocks be 

 compared with one from the shores of New Caledonia, there 

 is nothing to distinguish them externally save their colour. 



The principal structures in the Mollusca, both internal and 

 external, will now be treated of, and the comparative method 

 adopted, thereby avoiding some repetition, as many parts 

 correspond more or less in the different members of the 

 group. I shall lay particular stress upon those which are 

 important in discriminating fossil shells. 



Moixusca. 

 The Mollusca, or soft-bodied animals are divided into four 

 classes — the Lamellibranchiata, or Acephala {i.e., headless 

 molluscs), 1 the Scaphopoda, the Gastropoda, and the Cephalo- 

 poda. The name of the first is taken from the prevailing 

 form of the gill-filaments, which grow together into lamellar, 

 or plate-like organs ; the others derive their names from the 

 contractile, muscular disc, known as the foot, which serves as 

 a locomotor organ in the Lamellibranchiata and Gastropoda, 

 and probably as a burrowing, or digging organ in the 

 Scaphopoda. In the Cephalopoda it is represented by the 

 " funnel." 



1 Called also Pelecypoda by some, thus bringing the nomenclature of 

 the group into harmony. 



