DISTRIBUTION OF MOLLUSCA IN TIME. 



417 



DEVELOPMENT OF FAMILIES, GENERA, AND SPECIES, 

 IN TIME. 



Ph 3 



Gk.ological 



Systems. 



f Cainbrian 

 (. Silurian 



Devonian 

 ( Carboniferous 

 ( Permian* 



Trias 

 f L. Jurassic 

 I U. Jurassic 

 ( L. Cretaceous 

 I U. Cretaceous 



f Eocene 



•j Miocene 



'- Pliocene 



Eecent 



Eecent & Fossil 



y 32 



Order nf a'p'pearance of tlie groups of Shells. The first and most important 

 point she'.vn in the preceding Tables, is the co-existence of the four principal 

 classes of testacea from the earliest period. The highest and the lowest groups 

 were most abundant in the palaeozoic age ; the ordinary bivalves and univalves 

 attain their climax in existing seas. If there be any meaning in this order of 

 appearance it is connected with the general scheme of creation, and cannot 

 be inquired into separately ; but it may be observed that the last-developed 

 groups are also the most typical, or characteristic of their class, (p. 61.) 



The Cephalopoda exhibit amongst themselves unmistakable evidence of 

 order in their appearance and succession. The tetrabranchiate group comes 

 earliest, and culminates about the period of the first appearance of the more 

 highly-organized cuttle-fishes.f The families of each di\dsion which are least 

 unlike {Orthoceratidce and BelemnitidcB) were respectively the first deve- 

 loped. 



"■■ Those genera are estimated as belonging to each Sj'stem Miiich occur in the 

 strata hoih above and below, as well as those actually found in it. 



t The PaLfoteuthis of Bronn (not D'Orb.) appears to be a fish-bone, from the equi 

 valent of the Old-red sandstone in the Eifel. 



