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 a 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.]

Amongst the Jurassic Univalves (Gasteropoda) there are many examples of the ancient and long-lived Pleurotomaria; but on the whole the Univalves begin to have a modern aspect.  The round-mouthed ("holostomatous"), vegetable-eating Sea-snails, such as the Limpets (Patellidoe), the Nerites (Nerita), the Turritelloe, Chemnitzioe, &c., still hold a predominant place.  The two most noticeable genera of this group are Cerithium and Nerinoea—­the former of these attaining great importance in the Tertiary and Recent seas, whilst the latter (fig. 169)