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] S ee 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. Plicatula, is another genus of this family, of which there are three British fossil species. A delicate shell, with slender depressed spines (_P. inflata_. _Foss. South D._ pl. xxvi.), occurs in the Chalk Marl. The recent species are natives of the seas of warm climates. Pecten.--The common scallop-shell will serve as a type of this genus. The animals of these shells, unlike the oysters, have the power of locomotion, and when in the water, may be seen moving with rapidity, and flapping their shells to and fro with great activity. Numerous species are found fossil. In the Pliocene, and other marine tertiary deposits, Pectens abound; in the White Chalk there are several elegant forms (see _Foss. South D._ plate xxv.); many kinds in the Oolite and Lias; and several in the Devonian strata. A large Mediterranean species (_Pecten Jacobæus_, _Ly._ p. 152) occurs in the Pliocene strata of Palermo, in every stage of growth, and as perfect as if recent. The Chalk and Shanklin sand contain a small inequivalved Pecten, the lower valve of which is convex, and pentangular, the upper flat, and both strongly ribbed, or pectinated; it is named _Pecten quinquecostatus_ (_Foss. South D._ pl. xxvi. _Ly._ p. 212); and in the cretaceous strata of North America a variety of this species is found.