Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/240

192 192 J. S. GARDNER ON BRITISH CRETACEOUS PATELLID^ ETC.

11. On British Cretaceous Patellidjs and other Families of Patelloid Gastropoda. By J. Starkie Gardner, Esq., F.G.S. (Read January 24, 1877.)

[Plates VII.-1X]

A study of this group of Cretaceous Gastropoda, commenced some twelve months since, brought to light unexpectedly so many forms new to our Cretaceous series, some of the genera to which they are referred having, indeed, never before been found fossil, that I have deemed the result of sufficient importance to form the subject of a paper which I now lay before this Society. Conical or patelloid shells make their appearance amongst the earliest known Mollusca ; and from them seem to have been differentiated the convoluted forms represented by Bellerophon : in the earliest times these two typical forms can hardly be separated one from the other, being linked together by spirally twisted capuliform shells. So impressed does Pictet seem to have been with this that he, in his great work the ' Terrains Cretaces de Ste. Croix,' includes Bellerophina in the Fissurellidae. The families, species of which are here noticed, have been unusually persistent in form, having come down, in some cases, almost unchanged, even specifically, to the present day. The aspect of the group is therefore more recent in appearance than that of any other group of Gastropods from rocks of the same age. Although a considerable advance is here made in our knowledge of these families, from these particular rocks, yet the number of species de- scribed from single specimens tells us unmistakably how much more remains to be learned, and the inference is forced upon us that we must still possess, though undiscovered, a far greater variety of representatives of these mollusca. By the absence of many genera from our Cretaceous rocks which are found in earlier rocks, such as Rimula, or in contemporaneous rocks of other countries, such as Fissurella, Parmopliorus, Infundibulum, Galerus, and the upper valves of Hipponyx, this inference is greatly strengthened.

The study of these families is perhaps of more importance to the geologist than that of most others of the Gastropoda, as the depth at which each form lived is approximately defined, and their pre- sence or absence is therefore of assistance in understanding the physical conditions under which marine strata have been deposited. For instance, from the complete absence of fissured forms from the Gault andGreensand of Polkestone, Cambridge, and Blackdown, while they are abundantly represented elsewhere in rocks of approximately the same age, we may infer that these seas were shallow. Further remarks are appended to some of the specific descriptions ; especially noteworthy are some remarkable cases of mimicry among the CalyptraBidae. A table of genera and species is appended. In this table the * signifies that the genus is altogether new in Cretaceous rocks ; f, new to Europe ; N, that the species is new. In the