Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/746

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.