﻿BRITISH FOSSIL CEPHALOPODA, 



237 



GENERAL OBSERVATIONS. 



The foregoing study naturally gives rise to certain observations, to obtain the 

 power of making which is indeed the incentive to undertaking it. So far as 

 a single group is concerned, we ought here to gain some insight into the laws which 

 govern the appearance and disappearance of forms of life, and into the nature of 

 those groups of individuals to which we assign the term " species." For this purpose, 

 it may be well to draw up the following condensed table, showing the growth, 

 culmination, and in some cases the decay, of the various genera and groups. 



Table showing the Distribution of the Genera and Groups. 



Gentjs or Group. 



o 



03 



a 



Eh 



6 

 '53 

 a 



3 



m 



a> > 

 fe o 



3 



Upper 

 Llandovery. 



Wenlock 

 Shale. 



M 6 



9 



° s 



"a °° 



1 § 



hi 2 



Upper 

 Ludlow. 



03 



1) 



a 

 o 



03 



Orthoceras . 

 Cyrtoceras . 

 Poterioceras . 

 Gomphoceras 

 Phragmooeras 

 Ascoceras 

 Nautilus 

 Trochoceras . 

 Lituites . 

 Ophidioceras 

 Goniatites . 









1 

 1 



9 



? 



1 



25 

 6 

 2 



1 



3 



2 



7 



1 



1 



2 



11 



2 



1 

 1 



29 

 4 



2 

 1 



2 

 4 



1 



15 

 3 



4 

 5 



2 

 5 



1 



31 



. 8 



8 

 3 

 1 



3 

 6 



2 



f 



31 

 2 



1 



3 

 1 



? 



1 



6 

 1 



Conici 

 Inflati . 

 Spirales . 

 Irregulares . 







2 



9 

 1 



31 

 3 

 3 

 2 



8 



1 



2 



13 



1 

 1 



33 

 3 



2 

 5 



18 

 9 

 2 

 6 



39 



12 



3 



11 



33 

 4 

 1 

 1 



6 

 1 



We notice that there are two maxima, both in this table and in the more 

 general one, corresponding respectively with the Bala and with the Lower Ludlow 

 Beds. This fact is so general, and the reason sufficiently plain in the character 

 and wide range of the beds, that we cannot conclude that there was a corresponding 

 real falling off in the variety of forms between these two epochs. It is obvious 

 however, from the fact that the species in the Wenlock Limestone are compara- 

 tively few when compared with those in the shales on either side of them, that the 

 Cephalopods of those days were not commonly frequenters of clear and shallow 

 waters, but were partly pelagic, and not uncommonly gregarious in more or less 

 turbid waters. 



