Geological Society of London. 377 



■what appears to be the present state of our knowledge of the general 

 stratigraphical distribution of the fossil Lamellibranchiata in Britain. 

 As a class, the Lamellibranchs are sparingly represented in the 

 Lower, and more numerously in the Upper Silurian group, and fall 

 off again in the Devonian ; they greatly increase in number in the 

 Carboniferous, become scanty in the Permian and Trias, and attain 

 their maximum development in the Jurassic rocks. They are also 

 largely represented in the Cretaceous and Tertiary series. The 

 stratigraphical distribution of the two great subordinate groups, the 

 Siphonida and the Asiphonida, corresponds generally with that of 

 the class ; the Siphonida predominate over the Asiphonida in Tertiary 

 formations, whilst the reverse is the case from the Cretaceous series 

 downwards. Nearly all the families of Lamellibranchs are repre- 

 sented in the Jurassic and Carboniferous rocks, and in the former 

 very largely. The author remarked especially on the great develop- 

 ment of the Aviculidas in Carboniferous times. 



Discission. — Mr. Etheridge, after noticing the importance of the paper, remarked 

 that possibly the great difference observed in the proportions of the Lamellibran- 

 chiata 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 in 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. Grifiiths and the Eev. Mr. Wiltshire 

 were doing the same work for the Gault. "What the late Mr. S. P. Woodward had 

 done as to the distribution of the different species of moUusks 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 extent, doing the 

 same for the Lamellibranchiata as 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 Carboniferous 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 Rhsetic 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 Lamelli- 

 branchiates 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 as 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 Lamellibranchiates preserved in the different formations. 



Mr. Charlesworth remarked on the occurrence of Trigonia in the Aiistralian 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 generaliza- 

 tions 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 would 

 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 Brachio- 

 , poda had beeu found in the older rocks, and of Lamellibrauchiata in the newer. But 



