Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/568

 ferous strata the Lamellibranchiata were obtaining a preponderance over the Brachiopoda. He accounted for their comparative absence in formations of other ages, especially between the Upper Silurian and Rhaetic beds, by the best-known areas of those periods having been mainly continental, or containing principally freshwater or inland-sea remains, so that the true marine fauna was absent. In Carboniferous times possibly the true relative proportions of the two forms had been preserved in the deposits.

Mr. Judd was doubtful as to the safety of placing too great reliance upon figures. He questioned whether some of the conclusions as to the great increase of Lamellibranchiata between the Carboniferous and Jurassic periods could be substantiated. Much depended on the amount of the rocks present in different countries, and the study bestowed on each. The conditions also for the preservation of the fossils might be more favourable at one time than another.

Mr. Carruthers considered the tables of the greatest value, as indicating the present state of our knowledge. He called attention to the difference of conditions under which different deposits had accumulated, which must have to some extent affected the proportion of Lamellibranchiata preserved in the different formations.

Mr. Charlesworth remarked on the occurrence of Trigonia in the Australian seas, and on there being varieties of form among specimens of existing species so great that if they were found fossil they might be regarded as of several species.

Mr. Hughes considered that the data were too incomplete to justify the generalizations of some of the previous speakers. It had been pointed out that whenever the Tables showed a very large number of Lamellibranchs from any formation, that formation had been carefully worked out by local observers ; and therefore he should like to know in each case the proportion the Lamellibranchiata bore to the total number of fossils found. It had been shown also that a larger proportion of Brachiopoda had been found in the older rocks, and of Lamellibranchiata in the newer. But in the older rocks whole genera of Lamellibranchs are confined to horizons and localities which are not cut off by stratigraphical breaks, such as would allow us to think it at all probable that they can be characterized by peculiar genera. He thought the scarceness and irregular occurrence of Lamellibranchs in the older rocks could be best explained on the supposition that those portions of the older deposits which were least favourable to Lamellibranchs happened to be those now chiefly exposed to our search, and that those few portions are only in part worked out.

Mr. Jenkins observed that in thick deposits there was a far greater likelihood of numerous forms being present than in thin ; for thickness meant time, and time meant variation.

Prof. Morris dissented from this view, as in. thin littoral deposits an enormous number of shells might be present, while in beds formed in deep sea they might be almost entirely absent.