.104 NOTES ON CARBONIFEROUS MOLLUSC A. 



GENERAL NOTES. 

 BY JAMES NEILSON. 



IN concluding my lists of the larger groups of the Carboniferous mollusca 

 some interesting points regarding their distribution may be mentioned. 



On certain horizons of both the Lower and Upper Limestone Series an 

 associated group of shells always appears in some of the shales and the clay 

 ironstones, and sometimes in the calmy or cement limestones. The following 

 are its most characteristic genera : Nucula, Nuculana, Ctenodonta, Paral- 

 lelodon, and Cypricardia, amongst the bivalves ; Plcurotomaria, Murchisonia, 

 Eiiomphalus, Macroc/ieilus, Naticopsis, Loxonema, Dentalium, and Bellerophon, 

 amongst the univalves ; and Nautilus, Cyrtoceras, Orthoceras, and Goniatites, 

 amongst the chambered shells. 



From these same strata Corals, Crinoids, Polyzoa, and most of the Brachio- 

 pods, are absent, or at least rarely present ; but, on the other hand, in the 

 strata where these latter groups do occur abundantly, the above-mentioned 

 group of mollusca rarely appears. It may also be stated that the above genera 

 of mollusca are generally absent from the purer limestones, or are sparely 

 represented by different species which do not appear in the shales. A 

 cursory examination of any collection of Western Scottish fossils will show 

 that all the genera named are chiefly derived from the Lower and Upper 

 Limestone shales. 



This grouping of the molluscan and other marine faunas of the Scottish 

 Carboniferous sea-bottom has in all probability depended greatly upon the 

 depth of water, the temperature, and the nature of the sediments which 

 were then being deposited. The reappearance at wide intervals, and at several 

 horizons, of strata containing such varying groups of molluscan life shows 

 clearly that certain conditions of the sea-floor were requisite for their growth 

 and development, and also, on the other hand, that the same conditions were 

 not favourable to the growth of Corals, Crinoids, Polyzoa, and many of 

 the Brachiopods. 



