_Middle Purbeck._--To these succeed the Middle Purbeck, about 30 feet thick, the uppermost part of which consists of freshwater limestone, with cyprides, turtles, and fish of different species from those in the preceding strata. Below the limestone are brackish-water beds full of _Cyrena_, and traversed by bands abounding in _Corvulae_ and _Melaniae_. These are based on a purely marine deposit, with _Pecten_, _Modiola_, _Avicula_, and _Thracia_, all undescribed shells. Below this, again, come limestones and shales, partly of brackish and partly of freshwater origin, in which many fish, especially species of _Lepidotus_ and _Microdon radiatus_, are found, and a reptile named _Macrorhyncus_. Among the mollusks, a remarkable ribbed _Melania_, of the section _Chilira_, occurs. Immediately below is the great and conspicuous stratum, 12 feet thick, long familiar to geologists under the local name of "Cinder-bed," formed of a vast accumulation of shells of _Ostrea distorta_ (fig. 240.). In the uppermost part of this bed Mr. Forbes discovered the first echinoderm as yet known in the Purbeck series, a species of _Hemicidaris_, a genus characteristic of the Oolitic period. It was accompanied by a species of _Perna_. Below the Cinder-bed freshwater strata are again seen, filled in many places with species of _Cypris_, _Valvata_, _Paludina_, _Planorbis_, _Lymnea_, _Physa_, and _Cyclas_, all different from any we had previously seen above. Thick siliceous beds of chert, filled with these fossils, occur in a beautiful state of preservation, often converted into chalcedony. Among these Mr. Forbes met with gyrogonites (the spore vesicles of _Charae_), plants never before discovered in rocks older than the Eocene. Again, beneath these freshwater strata, a very thin band of greenish shales, with marine shells and impressions of leaves, like those of a large _Zostera_, succeeds, forming the base of the Middle Purbeck. [Illustration: Fig. 240. Ostrea distorta. Cinder-bed.]