Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/567

 but when we ascend to the upper division and to the Upper Silurian rocks, Lamellibranchs are numerous. In the Devonian system a much smaller number of species than is yielded by Silurian rocks rewards our search. The Carboniferous formations, however, give us a greatly increased number, nearly four hundred species having been described as occurring in these rocks. The numbers of Permian and Triassic species are small ; but Jurassic strata have furnished us with upwards of a thousand species of Lamellibranchiata, the Jurassic epoch having witnessed the maximum development of the class. The number falls to about half in the Cretaceous system, and rises again in the Tertiary deposits, strata of the latter age having furnished to science between six and seven hundred species of fossil Lamellibranchs. (See Table III.)

We thus see that the Lamellibranchiata have had a great development in each of the three great epochs of geological time, the Paloeozoic, the Mesozoic, and the Cainozoic, and have undergone great modifications of form during the long period required for the deposition of the sedimentary portion of the crust of our globe. As in other classes of the animal kingdom, some genera have lived on through many of the great cosmical changes which our planet has seen, while other generic forms have existed during only a comparatively brief portion of the earth's history.

Many and grave questions are suggested by this fact, a fact well known to students of science, but the elucidation and explanation of which are at the present time sought for by our most earnest thinkers, and form the battle-ground, so to speak, for the contests of the ablest and most richly furnished intellects of our day and generation.

Discussion.

Mr. Etheridge, after noticing the importance of the paper, remarked that possibly the great difference observed in the proportions of the Lamellibranchiata in different formations might to some extent be due to our want of knowledge. Of late years, in the Caradoc and Lower Silurian series, the number of species had been nearly doubled, principally through the persevering industry of one single observer, Lieut. Edgell. The same was to some extent the case with the Carboniferous rocks, owing to the collections of Mr. Carrington. Much was also being done for the Oolitic series, in connexion with which the names of Mr. C. Moore, Mr. Sharp, and Dr. Bowerbank ought to be mentioned. Mr. Griffiths and the Rev. Mr. Wiltshire were doing the same work for the Gault. What the late Dr. S. P. Woodward had done as to the distribution of the different species of mollusks through time, Mr. Lobley was doing on a larger and more extended scale.

Prof. Ramsay was glad to find that Mr. Lobley was, to some extent, doing the same for the Lamellibranchiata that Mr. Davidson had done for the Brachiopoda. He did not know how the case might be with the Silurian and Devonian formations ; but in the Carboni- VOL. XXVII. — PART I. 2 G