These shells are so intimately allied to the Hipintritidce, that Requienia has been frequently included with them in the apocryphal order Rudista" The members of the Hippu rite group are attached and gregarious, like oysters, often occurring in great numbers, and filling large tracts of rock. Their valves are different in structure and sculpturing, and are articulated by two prominent teeth above and one below ; the cartilage is internal, but there is a conspicuous ligamental furrow outside. There are nearly 100 species characteristic of the cretaceous strata, and especially of the lower chalk, or hippurite limestone." Only one species (Radiolites Mortoni) is found in England ; the rest are from the West Indies, Southern Europe, Algeria, and the East. The form which approaches nearest to Chanta is the little genus Caprotina (fig. 15, 7), whose upper valve has a marginal umbo, but is in other respects like a miniature Radiolite. Caprina (d'Orb.) has the free valve perforated by canals which open in the inner margin, and in Caprinella the outer lamina of both valves possesses this structure. One valve is sometimes spiral (fig.
15, 6), and partitioned of internally by numerous septa, like the water-Spondylus, but so regularly as to resemble the chambered shell of a Nautilus. In the Radiolite (fig. 15, s), Secondary Bivalves.
Diceraa arietinum, Lam.; Corallian, France.
2. Requienia ammonia ; Neocontian, S. France.
3. Monopleura trilobata, d'Orb.; Neocotnian, Orgon.
4. Hippurites 'roucaeiana, d'Orb. ; L. Chalk, France.
5. Radiolitee angeiodee, Lam.; L. Chalk, Gomm.
6. Caprinella Boiaayi, d'Orb ; L. Chalk, Valley of Alcantara.
7. Caprotina semietriata, d'Orb.; U. Greenaand, Le Mang.
both valves are conical, and the umbo of the free valve (mar ginal in the very young shell) becomes central in the adult. The structure of the hinge is modified by the absence of any spirality in the valves, but is essentially the same as in Capro tina and Dieeras ; the prominent teeth of the upper valve support curved plates for the attachment of the adductor muscles, which become continually more undercut in the course of their growth. In Hippurites the anterior muscular plate projects horizontally, the posterior vertically, like a third tooth, for which it has been mistaken. In this genus there are two longitudinal inflexions of the outer shell-wall beside the ligamental furrow, one corresponding to the posterior muscular plate, the other (or third) apparently a siphonal inflexion like that in Trigonia and Leda (fig. 15, 4).
The cockle-shells (Cardiadee), as they have a world-wide distribution now, bad a corresponding range in time, and are found in all strata from the Silurian upwards. The com monest fossil type of Cardium is ribbed concentrically on the sides, and radiately on the posterior slope, a style of ornament almost unique amongst the 200 recent species. The Caspian
cockles, distinguished by a sinus in the pallial line, appear to have inhabited the Aralo-Caspian region almost from the middle tertiary period ; the hinge-teeth are reduced to one (Monodacna) or two (Didacna) in each valve, and are some times quite wanting even in the young shell (Adacna, Eichw.) Lithocardium aviculare (fig. 16, 7) is a characteristic shell of the Paris basin, and appears to have spun a byssus, like the fry of some recent cockles ; it also resembles the oriental Tridaena, of which a species is found in the miocene of Poland. The genus Conocardium (fig. 13, s) of the upper Silurian and carboniferous systems is remarkable for the prismatic cellular structure of its shell, and the truncation of the posterior (?) side of the valves, which are furnished in some species with a slender siphonal process.
The fresh-water Cycladidce are represented in the Wealden and eocene by many species of Cyrena, mostly of small size. The recent Corbicula fluntizzalis of eastern rivers is a common fossil of the Pliocene tertiary in England and Sicily.
The Cyprinidce and Astartidce are more abundant as fossil shells, and had a wider range of old than at the present day. Nearly 100 species of Cyprina have been catalogued, com mencing in the trias ; the dentition of the older species is, however, somewhat peculiar. The Isocardice are almost as numerous, and have the same range, but many of the fossil Isocardia-looking shells are really related to the Anatinicke. A yet higher antiquity has been assigned to Cypricardia, a genus now very scarce and difficult to obtain, on account of its habit. The paheozoic Pleurophorus (fig. 13, 9) is dis tinguished by the prominent ridge behind the anterior muscular impression ; and Meyalodon (J. Sby.), by the plate supporting the posterior adductor. This genus is represented in the oolites by Pachyrisma (fig. 16, 1), and in the tertiaries and modern seas by Cardilia.