1871.] LOBLET — BRITISH LAMELIIBRANCHIATA. 417 



but when we ascend to the upper division and to the Upper SUurian 

 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 Per- 

 mian and Triassic species are small ; but Jurassic strata have 

 furnished us with upwards of a thousand species of Lamellibran- 

 chiata, the Jurassic epoch having witnessed the maximum develop- 

 ment 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 spe- 

 cies of fossil LameUibranchs. (See Table III.) 



We thus see that the Lamellibranchiata have had a great develop- 

 ment in each of the three great epochs of geological time, the 

 Paheozoic, the Mesozoic, and the Cainozoic, and have undergone great 

 modifications of form during the long period required for the depo- 

 sition 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 compa- 

 ratively brief portion of the earth's history. 



Many and grave questions are suggested by this fact, a fact well 

 known to students of science, biit 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. 



DiSCTTSSION, 



Mr. Etheridge, after noticing the importance of the paper, re- 

 marked that possibly the great difference observed in the propor- 

 tions 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. EdgeU. The same was to some extent the 

 case with the Carboniferous rocks, owing to the collections of Mr. Car- 

 rington. Much was also being done for the Oolitic series, in con- 

 nexion with which the names of Mr. C. Moore, Mr. Sharp, and 

 Dr. Bowerbank ought to be mentioned. Mr. Griffiths and the 

 Eev. 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. Eamsay was glad to find that Mr. Lobley was, to some ex- 

 tent, 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. 



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