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. 



