Lign. 174. Fossil Cyprides. Wealden. Sussex and Isle of Wight.
| Fig. | 1.— | Cypris Valdensis: magn.; showing the ventral aspect of the united valves. Cooksbridge. |
| 1a.— | Side view of the same, magn. The three small figures areof the nat. size. | |
| 2.— | Cypris Fittoni, magn. Langton Green. | |
| 3.— | Cypris tuberculata, magn. Langton Green. | |
| 3a.— | View of the same, showing the lower edges of the valves united: the three small figures, nat. | |
| 4.— | Cypris granulosa, magn.; view showing the hinge-line. | |
| 5.— | A piece of Weald Clay, with Cyprides and Shells; nat. |
Cypris. [Lign. 174].—The animals of this genus belong to those Crustaceans in which the covering of the body is not divided into transverse segments, but consists of a large dorsal shield, having the form of a bivalve shell. They are often very minute, and numerous kinds swarm in our lakes and pools. The species of an allied genus, named Cythere, the shells of which cannot always with certainty be distinguished in a fossil state from those of Cypris, inhabit salt or brackish waters. As the living Cyprides are interesting objects for microscopic examination, they are commonly shown in the exhibitions of the hydro-oxygen microscope, and their appearance is therefore well known. Two recent species are figured in Ly. p. 183. These animals have the body enclosed in a bivalve, horny case, the two pieces being united by a hinge-line. They have four feet, and two pairs of antennæ, with a pencil of cilia at the extremities; and one compound eye. They swim with rapidity, by means of their ciliated antennæ, and crawl about actively on the water-weeds and other subaqueous surfaces. Like the other crustaceans, they frequently moult or cast their cases, and the surface of the mud spread over the bottoms of still lakes is often covered with their exuviæ. The largest living Cypris (C. clavata) does not exceed one-eighth of an inch in length. The fossil cases or shells of Cyprides are found in considerable variety and in prodigious numbers in certain Tertiary and Secondary strata, which appear to have been deposited by fresh-water; as, for example, in the lacustrine marls of Auvergne (Ly. p. 183), and the fluviatile clays and limestones of the south-east of England. They have not been observed in any decidedly marine beds; but Mr. Lonsdale discovered in the Chalk, by means of the microscope, cases of crustaceans, that belong to the genus Cythere (Ly. p. 26, figs. 21-24), the recent species of which inhabit the sea. One or more species of Cypris have been observed in the older British Tertiaries,—by Sir C. Lyell, in Hordwell Cliff (Geol. Trans. 2d ser. vol. iii. p. 288), and by Mr. Prestwich at Hempstead Cliff (Brit. Assoc. 1846); but in the Pleistocene deposits the Cypris and its congeners abound, and of these Mr. R. Jones has enumerated nine species (Annals Nat. Hist. 2d ser. vol. iii.). In many districts on the Continent, the Eocene marls and clays abound in these remains. Some of the fresh-water Tertiary strata of France contain myriads of a Cypris (named C. faba, from its bean-like form) which was formerly supposed, but erroneously, to be identical with a species found in the Wealden. The laminated marls of Auvergne contain, between every layer, countless myriads of the shells of Cyprides, through a depth of several hundred feet; although each lamina of marl scarcely exceeds the thickness of paper. Near Clermont, the green cypridiferous marls approach to within a few yards of the granite which forms the borders of that ancient basin (Ly. p. 184). In the eastern portion of the basaltic districts of India, Mr. Malcolmson has discovered two species of Cypris, associated with fresh-water univalves and bivalves.[461] In England the principal deposits of the Cyprides, are the clays and limestones of the Wealden and the Isle of Purbeck,[462] to the composition of which the relics of these minute beings have largely contributed. Entire layers of their cases are found in the laminated clays and marls on the southern shores of the Isle of Wight, at Atherfield and Sandown Bay, where some of the Wealden beds emerge from beneath the lower division of the Shanklin sand. Upon splitting the clay in the direction of the laminæ, the exposed surfaces are found to be covered with these minute bodies; as in the specimen, [Lign. 174], fig. 5.
[461] Geol. Trans. 2d ser. vol. v. pl. xlvii.
[462] On account of the distinctive characters of their carapace-valves, M. Bosquet has suggested the appellation Cypridea as a generic name for the Cyprididæ of the Wealden and Purbeck beds.
The appearance of four characteristic Wealden Cyprides, of their natural size and magnified, is shown in [Lign. 174]. The one named C. Valdensis, or Wealden Cypris, by Dr. Fitton and Mr. Sowerby (fig. 1), is the most frequent, and occurs in numerous localities in Kent and Sussex. A Cypris having the case studded with relatively large tubercles (fig. 3), is found in many of the finer sandstones of Tilgate forest; another, with the shell tuberculated, but divided by a transverse ridge (fig. 2), indicating a rudimentary condition of the segments which characterise the class, is certainly distinct from the former, and can have no name more appropriate than that of C. Fittoni. The other beautiful species (C. granulosa, fig. 4) has the surface of the case covered with granules. One more species has been observed in the Weald clay, at Sandown Bay and Atherfield (by Mr. Lonsdale); it is distinguished by a short conical spine on each valve, and is therefore named C. spinigera. In the Weald clay at Resting-oak-hill, near Cooksbridge (Geol. S. E. p. 187), C. Valdensis is so abundant, that every thin flake is covered with its white calcareous shells; and upon breaking the nodules and septaria of reddish-brown ironstone which occur in that locality, myriads of beautiful sharp casts of the cases are observable in almost every fragment. They are associated with fresh-water bivalves and univalves (Cyclas and Paludina), and minute scales of fishes. The sandstone at Langton Green, near Tunbridge Wells, which contains casts and impressions of several species of fresh-water shells, abounds in Cyprides; and the layers of argillaceous ironstone, interstratified with the sandstone in one of the quarries, are particularly rich in these remains. The surface of a recently broken slab is often covered by minute, polished, oblong, convex bodies, which are the casts of cypridean carapaces.
The Sussex marble is largely composed of the remains of these minute crustaceans. Upon examining thin polished slices of this limestone under the microscope, the cavities and interstices of the shells are found to be filled with the shields of Cyprides, entire or in fragments; and some specimens of the Purbeck marble equally abound in these remains. The Purbeck marls, as well as the limestones, often abound with Cyprides. According to Prof. E. Forbes, these belong to several species, all more or less distinct from those of the Wealden (Brit. Association, 1850). The Cyprides of the Wealden of Germany have been figured and described by Dr. Dunker; and several of the British forms are found there.
As the recent species inhabit still lakes, or gently running streams, and not the turbulent waters of estuaries, we cannot doubt that the strata in which these animals so largely predominate were deposited in lakes or bays, communicating with the river which transported to their present situation the bones and other remains of the colossal reptiles of the Wealden. And the beds of fresh-water snails, with scarcely any intermixture of other organic remains but the Cyprides, which are spread over extensive areas in the Wealden and Purbeck districts, appear to afford corroborative proof of this inference.
Four species of minute bivalved Entomostraca from the Carboniferous deposits have been referred to Cypris: viz. C. arcuata, Bean, from the Coal-shale at Newcastle; C. inflata, Murchison, Coal-measures, near Shrewsbury; C. Scoto-Burdigalensis, Hibbert, Coal-measures at Burdie-house, and Coal-shale at Derry; and C. subrecta, Portlock, also from Derry, Tyrone.