■DISTEIBtJTION OF MOLLTTSCA LN" TIME. 



127 



DEVELOPMENT OF FAMILIES, GENEEA, AND 

 SPECIES, IN TIME. 



Geological 

 Systems. 



, jCambrian 



'(Silurian 



2 Devonian 



Q j" Carboniferous 



(Permian* 



4 Trias 



- J L. Jurassic 



1 U. Jurassic 



^ JL. Cretaceous 



(U. Cretaceous 



^Eocene 



7<Miocene 



(Pliocene 



Kecent 



Pvecent & Possil 



Total 



of 



Genera. 



49 



79 

 6(5 



81 

 107 

 108 

 123 



143 



172 

 173 

 192 



400 



520 



56 280 



85 



97 



100 



251 



34 



76 



79 



115 



150 



Total 

 Number of 



Species 

 (D'Orbigny), 



362 



317 



1035 



835 

 74 



713 

 1502 

 12.66 



784 

 2147 



2636 



2242 



• 437 



16,000 



a 



30,000 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 festacea from the earliest 

 period. The highest and the lowest groups were most abundant 

 in the palaeozoic age ; the ordinarj^ 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 arc also the 

 most typical, or cJiaracferistic of their class (p. 49). 



The Cephalopoda exhibit amongst themselves unmistakable 

 evidence of order in their aj)pearance and succession. The 

 tetrabranchiate group comes earliest, and culminates about the 

 period of the first appearance of the more highly- organised cuttle- 

 fishes.! The families of each division which are least unlike 



* Those genera are estimated as belonging to each sj'stera which occur in tlie strata 

 both above and below, as well as tiose actuallj' found in it. We liave left this table as 

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

 is not of much importance, since the main points, such as the gi-adual increase in the 

 number of families, would not be affected. 



t The Palceoteuthis of Brcnn (notD'Orb.) appeals to be a. fish-bone, from the equ>a^ 

 tut of the Old Red sandstorDJ in the Eifel. 



