Fig. 164.—Eryon arctiformis, a "Long-tailed Decapod," from the Middle Oolites (Solenhofen Slate). and beautiful as they are, must be at once dismissed; but the Brachiopods deserve a moment's attention. The Jurassic Lamp-shells (fig. 165) do not fill by any means such a predominant place in the marine fauna of the period, as in many Palæozoic deposits, but they are still individually numerous. The two ancient genera Leptœna (fig. 165, a) and Spirifera (fig. 165, b), dating the one from the Lower and the other from the Upper Silurian, appear here for the last time upon the scene, but they have not hitherto been recognised in deposits later than the Lias. The great majority of the Jurassic Brachiopods, however, belong to the genera Terebratula (fig. 165, c, e, f) and Rhynchonella (fig. 165. d), both of which are represented by living forms at the present day. The Terebratulœ, in particular, are very abundant, and the species are often confined to special horizons in the series.
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
Fig. 165.—Jurassic Brachiopod. a. Leptœna Liassica, enlarged, the small cross below the figure indicating the true size of the shell—Lias; b, Spirifera rostrata, Lias; c, Terebratula quadrifida, Lias; d, d', Rhynchonella varians, Fulter's Earth and Kelloway Rock; e, Terebratula sphœroidalis, Inferior Oolite; f, Terebratula digona, Bradford Clay, Forest-marble, and Great Oolite. (After Davidson). 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 Cyrenœ and Corbulœ. 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 Gryphœa (fig. 167). In the former of these are included Oysters with the beaks
Fig. 166.—Ostrea Marshii. Middle and Lower Oolites.
Fig. 167.—Gryphœa incurva. Lias. "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 Exogyrœ is the E. Virgula of the Oxford Clay, and of the same horizon on the Continent; and the Gryphœa 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
Fig. 168.—Diceras arietina. Middle Oolite. 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.