418 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [May 24, 



ferous strata the Lamellibranehiata were obtaining a preponderance 

 over the Brachiopoda. He accounted for their comparative absence 

 in formations of other ages, especially between the Upper Silurian 

 and Ehsetic 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 reli- 

 ance upon figures. He questioned whether some of the conclusions 

 as to the great increase of Lamellibranehiata between the Carbo- 

 niferous and Jurassic periods could be substantiated. Much de- 

 pended on the amount of the rocks present in different countries, 

 and the study bestowed on each. The conditions also for the pre- 

 servation of the fossils might be more favourable at one time than 

 another. 



Mr. Caekuthees considered the tables of the greatest value, as 

 indicating the present state of our knowledge. He called attention 

 to the diiference of conditions under which diiferent deposits had 

 accumulated, which must have to some extent affected the propor- 

 tion of Lamellibranehiata preserved in the different formations. 



Mr. Chaklesworth remarked on the occurrence of Trigonia in the 

 Australian seas, and on there being varieties of form among spe- 

 cimens 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 Lamellibran- 

 ehiata 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 Lamellibranehiata 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 cha- 

 racterized by peculiar genera. He thought the scarceness and irre- 

 gular 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. Jenkiks 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. 



