The stone called “Purbeck Marble,” formerly much used in ornamental architecture in the old English cathedrals of the southern counties, is exclusively procured from this division. Fig. 299: Cyprides from the Upper Purbecks. _Middle Purbeck._—Next in succession is the Middle Purbeck, about thirty feet thick, the uppermost part of which consists of fresh-water 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 _Corbula_ and _ Melania._ These are based on a purely marine deposit, with _ Pecten, Modiola, Avicula,_ and _Thracia._ Below this, again, come limestones and shales, partly of brackish and partly of fresh-water origin, in which many fish, especially species of _ Lepidotus_ and _Microdon radiatus,_ are found, and a crocodilian reptile named _Macrorhynchus._ Among the mollusks, a remarkable ribbed _Melania,_ of the section _Chilina,_ occurs. Fig. 300: Ostrea distorta. Fig. 301: Hemicidaris Purbeckensis. Immediately below is a great and conspicuous stratum, twelve feet thick, formed of a vast accumulation of shells of _Ostrea distorta_ (Fig. 300), long familiar to geologists under the local name of “Cinder-bed.” In the uppermost part of this bed Professor Forbes discovered the first echinoderm (Fig. 301) as yet known in the Purbeck series, a species of _ Hemicidaris,_ a genus characteristic of the Oolitic period, and scarcely, if at all, distinguishable from a previously known Oolitic fossil. It was accompanied by a species of _Perna._ Below the Cinder-bed fresh-water strata are again seen, filled in many places with species of _Cypris_ (Fig. 302, _a, b, c_), and with _Valvata, Paludina, Planorbis, Limnæa, Physa_ (Fig. 303), and _Cyclas,_ all different from any occurring higher in the series. It will be seen that _Cypris fasciculata_ (Fig. 302, _b_) has tubercles at the end only of each valve, a character by which it can be immediately recognised. In fact, these minute crustaceans, almost as frequent in some of the shales as plates of mica in a micaceous sandstone, enable geologists at once to identify the Middle Purbeck in places far from the Dorsetshire cliffs, as, for example, in the Vale of Wardour in Wiltshire. Thick beds of chert occur in the Middle Purbeck filled with mollusca and cyprides of the genera already enumerated, in a beautiful state of preservation, often converted into chalcedony. Among these Professor Forbes met with gyrogonites (the spore-vessels of _Chara_), plants never until 1851 discovered in rocks older than the Eocene. About twenty feet below the “Cinder-bed” is a stratum two or three inches thick, in which fossil mammalia presently to be mentioned occur, and beneath this a thin band of greenish shales, with marine shells and impressions of leaves like those of a large _Zostera,_ forming the base of the Middle Purbeck. Fig. 302: Cyprides from the Middle Purbecks.