GENERAL CHARACTERS OF MOLLUSCA. 189 



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

 lustre, due to the minute undulations of the edges of alternate 

 layers of carbonate of lime and membrane. The " fibrous " 

 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 h'ard structures, whether 

 external or internal, no fossils are more abundant or import- 

 ant than Molluscs. As regards the general distribution of the 

 Mollusca in time, the sub-kingdom commences its existence in 

 the Cambrian period, and there is no reason to suppose that 

 this is really its first appearance. In the Cambrian Rocks, 

 the classes of the Polyzoa, Brachiopoda, Pteropoda, Gasteropoda, 

 and Cephalopoda are certainly represented, and the Lamelli- 

 branchiata existed in Lower Silurian times, if not earlier. 

 Speaking generally, the chief representatives of the Mollusca 

 in Palaeozoic time are the chambered Cephalopods (Tetra- 

 branchiata} and the Brachiopoda; in Mesozoic time, the Cuttle- 

 fishes (Dibranchiate Cephalopods), the chambered Cephalo- 

 pods, and the Polyzoa ; in Kainozoic time, the Lamellibranchs 

 and Gasteropods. The Polyzoa are comparatively poorly re- 

 presented in Palasozoic Rocks, and attain their maximum 

 towards the close of the Mesozoic period. The Brachiopods 

 are vastly more abundant in Palaeozoic deposits than in Meso- 

 zoic, and have gradually declined to the present day. The 

 Lamellibranchiata seem to have been gradually increasing in 

 importance since their first appearance in the Lower Silurian 

 seas, and they have attained their maximum at the present 

 day. The Gasteropods, upon the whole, like the Bivalves, 

 seem to have reached their culminating point in recent seas ; 

 whilst the Pteropods seem to have been as abundant in Silurian 

 seas as they are at present. The history of the Cephalopoda is 

 a remarkable one. The Tetrabranchiate forms, with chambered 

 shells, attained their maximum in the earlier portion of the 

 Silurian period, as regards their simpler types ; but the more 

 complex types of the group swarmed in the seas of the 

 Secondary period, and finally disappeared at the close of this 

 epoch. This group at the present day is represented solely 

 by the Pearly Nautilus. The Dibranchiate Cephalopoda, on 

 the other hand, represented at the present day by the Cuttle- 



