Remains of _Bivalves_ (_Lamellibranchiata_) are very numerous in the Jurassic deposits, and in many cases highly characteristic. In the marine beds of the Oolites, which constitute by far the greater portion of the whole formation, the Bivalyes are of course marine, and belong to such genera as _Trigonia, Lima, Pholadomya, Cardinia, Avicula, Hippopodium_, &c.; but in the Purbeck beds, at the summit of the series, we find bands of Oysters alternating with strata containing fresh-water or brackish-water Bivalves, such as _Cyrenoe_ and _Corbuloe_. The predominant Bivalves of the Jurassic, however, are the _Oysters_, which occur under many forms, and often in vast numbers, particular species being commonly restricted to particular horizons. Thus of the true Oysters, _Ostrea distorta_ is characteristic of the Purbeck series, where it forms a bed twelve feet in thickness, known locally as the "Cinder-bed;" _Ostrea expansa_ abounds in the Portland beds; _Ostrea deltoidea_ is characteristic of the Kimmeridge clay; _Ostrea gregaria_ predominates in the Coral-rag; _Ostrea acuminata_ characterises the small group of the Fuller's Earth; whilst the plaited _Ostrea Marshii_ (fig. 166) is a common shell in the Lower and Middle Oolites. Besides the more typical Oysters, the Oolitic rocks abound in examples of the singularly unsymmetrical forms belonging to the genera _Exogyra_ and _Gryphoea_ (fig. 167). In the former of these are included Oysters with the beaks "reversed"--that is to say, turned towards the hinder part of the shell; whilst in the latter are Oysters in which the lower valve of the shell is much the largest, and has a large incurved beak, whilst the upper valve is small and concave. One of the most characteristic _Exogyroe_ is the _E. Virgula_ of the Oxford Clay, and of the same horizon on the Continent; and the _Gryphoea incurva_ (fig. 167) is equally abundant in, and characteristic of, the formation of the Lias. Lastly, we may notice the extraordinary shells belonging to the genus _Diceras_ (fig. 168), which are exclusively confined to the Middle Oolites. In this formation in the Alps they occur in such abundance as to give rise to the name of "Calcaire à Dicerates," applied to beds of the same age as the Coral-rag of Britain. The genus _Diceras_ belongs to the same family as the "Thorny Clams" (Chama) of the present day--the shell being composed of nearly equally-sized valves, the beaks of which are extremely prominent and twisted into a spiral. The shell was attached to some foreign body by the beak of one of its valves. [Illustration: Fig. 168.--_Diceras arietina_. Middle Oolite.] [Illustration: Fig. 169.--_Nerinoea Goodhallii_, one-fourth of the natural size. The left-hand figure shows the appearance presented by the shell when vertically divided. Coral-rag, England.]