642 EDWAKD HULL ON THE CLASSIFICATION 



The remains of reptiles are too scarce and vague for such a purpose ; 

 and the like may be affirmed of the Crustacea, with the exception of 

 the Trilobites, whose presence in vast numbers amongst the marine 

 beds of the Silurian period places the nature of their habits beyond 

 question. We therefore fall back upon those orders and genera of 

 molluscs whose habits can be determined either by actual observa- 

 tion, or association with other forms, as guides to classification of 

 strata. 



The lacustrine conditions of the Old Red Sandstone with Anodonta 

 Jukesii in Ireland gradually give place to marine conditions in the 

 period of the Lower Carboniferous Slate, which in other districts 

 (except in Scotland, where the beds are marginal) extended over 

 the remainder of the British Isles as far as they were submerged. 

 Throughout the period of the Carboniferous Limestone deep-sea con- 

 ditions generally prevailed, and the molluscan fauna flourished to 

 an extent greater than at any subsequent stage of the Carboniferous 

 period. 



With the close of the period represented by the Carboniferous 

 Limestone the seas became shallower, and muddy sediment pervaded 

 the heretofore clear waters. The marine fauna became dwarfed, or 

 carried on a lingering existence, and in numerous instances died out 

 altogether. Those forms which survived sometimes became modified 

 through the succeeding periods ; but, as may be seen from the list 

 of species (Table II.), a goodly number reached the stage of the 

 Gannister beds ; and probably, were we fully acquainted with the 

 entire fauna of the Carboniferous Limestone seas, we should find 

 that all the forms in Stage E have descended from the earlier 

 period. 



Amongst the known and prevalent marine forms in the Gannister 

 beds are a few of those bivalves to which Professor Xing has given 

 the name of " Antliracosia." Their presence probably indicates the 

 recurrence of estuarine conditions from time to time at this stage ; 

 but subsequently they assumed a high importance as the represen- 

 tatives of molluscan life during the periods of the Middle and Upper 

 Coal-measures, amongst the beds of which formations they occur at 

 intervals throughout. That these shells were sometimes estuarine, 

 sometimes lacustrine, appears to be borne out by the evidence before 

 us, especially their rare association in the same stratum with truly 

 marine forms of the succeeding stages. 



(b) Census of Marine Forms (British Islands). 



This decided change in the fauna of Stage E (Gannister beds), as 

 compared with that of Stages F and G, will be recognized upon 

 reference to the Tables of Species, which have been drawn up, with 

 the assistance of Mr. Baily, E.G.S., from available sources. Erom 

 these we find : — 



1 . That Stage E (Gannister beds) has yielded 36 genera with 

 about 70 species, of truly marine forms. 



