Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/254

206 206 j. s. gardner on british cretaceous tatellid^e etc.

Discussion.

Prof. Seeley remarked that the paper raised questions both large and small. The author's inferences w'.'h regard to depth of seas and questions of old physical geography were of great importance, if they could be established. He infers, from the presence of cer- tain shells, that the Gault was deposited in shallow, and the Green- sand in deep water; but this was opposed to all evidence of a phy- sical kind, and was therefore untrustworthy. Coarse deposits are formed near the shore, and finer ones at greater distances ; there- fore the Gault must have been formed in deeper water than the Upper Greensand. Prof. Seeley also maintained that the occurrence of similar species did not necessarily imply similarity of conditions, and that therefore no inference as to physical conditions could be drawn from extinct species of marine fossils.

Mr. Charlesworth made a comparison of the old limpets with those of the present day, and, in connexion with the author's re- marks upon the apparent mimicry presented by the Cretaceous species, stated that neither in the later Tertiary deposits which he had especially studied, nor on existing beaches, had he ever found a limpet bored by carnivorous Gastropoda after their well-known fashion.

Mr. Meyer said that, with respect to the comparative depths of the Gault and Greensand seas, he agreed with the author, for two reasons : — one derived from the fauna, namely that Brachiopoda, which are usually held to be inhabitants of deep water, abound in the Greensand, but are almost entirely absent in the Gault ; the other from the mineral condition of the two deposits, as he believed that while the immediate shore-line of the Greensand was rocky, in part Portlandic, yielding sand rather than clay, the shore-line of the Gault was mainly Kimmeridgian, yielding clay rather than sand. This would partly account for the difference in mineral condition.

Mr. Price said that in estimating from palaeontological data the approximate depth of the Gault sea, he found that the lower part of the formation was deposited in shallow water, the middle in deeper, and the upper part in still deeper water, until the oceanic conditions of the Chalk were nearly reached.

The Author, in reply, said that he could not agree with Prof. Seeley. The Greensand of Cambridge and Blackdown possess none of these Mollusca ; and the Greensand of Warminster, with its numerous Echini, is certainly a deep-water formation. The Gault is known to be a shallow-water deposit. At the same time it was to be admitted that our knowledge of the Cretaceous Limpets is ex- ceedingly imperfect, a great number of the species described being founded on unique specimens. He regarded the Limpets as repre- senting the most ancient form of the Gastropod shell, the simple capuliform shell gradually passing into the convoluted form, such as Bellerophon, which Pictet had included among the Limpets. The persistence of the type is therefore exceedingly remarkable.