Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/249

Rh J. S. 6AED2SBB ON BRITISH CRETACEOUS PATELLTD^ ETC. 201

CALYPTRJEIDJE.

Calyptr^a Cooksoxle, Secley. Gault.

Shell elevated, cap-shaped, wider than high, longer than wide, inflated, rounded anteriorly, flattened posteriorly ; apex posterior, recurved; septal scar variable, rectangular or U-shaped, narrow. The cast only is known, which is marked by fine lines of growth.

This shell is known only from Cambridge, where it is frequently found inside the last chambers of Cephalopoda.

It was figured in 1861 by Seeley, in the ' Ann. and Mag. of Nat. Hist.' as a Crepidula, and almost at the same time by Pictet and Campiche as Calyptrcea sanctce-crucis. Seeley's name would seem to have priority, as that part of Pictet's work was published after 1861.

The form of the sear indicates a relationship with Calyptrcea rather than Crepidula. Pictet's figures, if really of one species, show the shell to be variable in height and in the form of the scar. Further specimens may unite it with Crepidula gaultina, Buv.

Calyptr2ea coxcextrica, sp. liov. Gault. PI. VII. figs. 27-30.

Unsymmetrical, cap-shaped, irregular, elevated ; sulcated con- centrically, like an Tnoceramus, without radiating marks ; apex re- curved, acute, projecting beyond, and overhanging, posterior margin.

This shell bears so great a resemblance to Inoceramus concent ricus that it has been hitherto overlooked by all collectors. The mimicry by this and other Calyptraeidae of the prevailing Monomyarian bi- valves of the age in which they lived, is continued at the present day, as in the cases of Calyptrcea niveata, Lessoni, plana, dilatata, squa- mosa, Jimbr lata, and many others, which can hardly be distinguished from small species of oysters. This mimicry, we may suppose, is protective, and enables the mollusk, which from the thinness of its shell might otherwise be selected by the boring carnivora in pre- ference to the thicker-shelled Ostrece, to escape. Other deep-water limpets are peculiarly open to attacks of this kind.

Specimens are not uncommon in the nodule-bed of the Gault at Folkestone.

Calyptrjsa Grayana, Tate. Chalk. PL VII. fig. 25.

" Shell ovate, conical, elevated, summit slightly recurved, orna- mented by numerous angular raised ribs, decussated by flexuous and inequidistant lines of growth."

The description is taken from the 'Quart. Journ.' vol. xxi. (I860), p. 38, pi. iii. figs. 8, 8 b. These figures are twice nat. size, and are inaccurate in this respect, that they represent the lines of growth which decussate the ribs too regularly arranged, there being in reality only occasional and very irregular lines of growth. The ribs or, more properly, striag are almost invisible without the aid of a lens, there being as many as thirty to every 5 millims. breadth of shell, with interspaces between them of about three times the