"DISTRIBUTIOIf OF MOLLUSCA IN TIME. 



127 



DEVELOPMENT OF FAMILIES, GENEEA, AND 

 SPECIES, IN TIME. 



Geological 



Systems. 



, (Cambrian 



'(Silurian 



2 Devonian 



o J Carboniferous 



(Permian* 



4 Trias 



c jL. Jurassic 



(U. Jurassic 



« j L. Cretaceous 



(U. Cretaceous 



^Eocene 



7-|Miocene 



(Pliocene 



Recent 



Eecent & Fossil 



Total 



of 



Genera. 



49 

 53 



77 

 79 

 66 



81 

 107 

 108 

 123 

 148 



172 



178 

 192 

 400 



520 



56 



85 

 97 



loa 



251 



280 



34 



72 



76 



79 



115 



150 



Total 

 Number of 



Species 

 (D'Orbigny). 



362 



317 



1035 



835 

 74 



713 



1502 

 1266 



784 

 2147 



2636 



2242 



437 



16,000 



30,000 



a 



85 \ 



42 



49 



62 



56 



18 



20 



24 1.32 



30 



30 



57 



60 



78 

 85 



Order of Appearance of the Groups of Shells. — The first and most 

 important point shown 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 oj their class (p. 49). 



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-organised cuttle- 

 fishes, f The families of each division which are least unlike 



* Those genera are estimated as belonging to each system which occur in the strata 

 both above and below, as well aa *Jiose actually found in it. We have left this table as 

 it stood in the first edition, as we are unable to correct aU the figures. This, however, 

 is not of much inaportance, since the main points, such as the gradual increase in the 

 number of families, would not be affected. 



t The Palceoteuthis of Eronn (notD'Orb.) appears to be s, fish-bone, from the equ'va* 

 ent of the Old Eed sandstone in the Eifel. 



