■DISTRTBTTTION OF MOLLTJSCA IN" TIME. 



127 



DEYELOPMENT OF FAMILIES, GENERA, AND 

 SPECIES, IN TIME. 



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 as *bose actually found in it. We have left this table aa 

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

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

 number of families, would not be affected. 



t The Palaoteuthis of Bronn (notD'Orb.) appears to be & fish-bone, from the eqnva^ 

 ent of the Old Red sandstone in the Eifel. 



