Lign. 128. Spondylus spinosus. In Chalk-flint. Lewes.
In the cretaceous strata of North America, Dr. Morton has discovered a Spondylus (S. dumosus) very nearly related to S. spinosus; but it differs in its general form, and has both valves beset with strong spines. I have the fragment of a large bivalve from the Kentish Rag (Mr. Bensted's quarry), which has the peculiar structure of the Water-clam (Spondylus varius of Mr. Broderip); namely, hollow interspaces formed by shelly layers or partitions, which were secreted by the posterior part of the mantle, or investing integument of the animal, as it gradually receded from that part of the shell. In the recent Water-clam the cells are full of fluid.[359]
[359] See Penny Cyclop. Art, Spondylidæ.
Plagiostoma, Llhwyd, 1699. This genus, adopted by Mr. Sowerby in the Mineral Conchology, is scarcely distinguishable from Lima of Bruguiere (1791). Most of the recent species are ornamented with small asperities, from which the name lima (file) is derived; they are symmetrical shells attached by a byssus.
Several smooth species of this genus are found in the Chalk,[360] Oolite, and Lias. A very large species (P. giganteum), sometimes ten inches in diameter, abounds in the Lias (Ly. p. 274). It is somewhat depressed in form, with the surface slightly striated; each valve has a pointed beak, with two lateral expansions, or ears, as they are termed by conchologists.
[360] See Foss. South Downs,, plate xxvi.