380 



MIDDLE PUBBECK. 



[Ch. XX 



The stone called " Purbeck marble," formerly much used in ornamental 

 architecture in the old English cathedrals of the southern counties, is ex- 

 clusively procured from this division. 



Middle Purbeck. — Next in succession is 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 pre- 

 ceding 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, Thracia, all undescribed shells. Below this, again, come lime- 

 stones 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 crocodilian reptile named Macrorhyncus. Among the 

 mollusks, a remarkable ribbed Melania, of the section Chilina, occurs. 



Immediately below is the great and conspicuous stratum, 1 2 feet thick, 

 long familiar to geologists under the local name of " Cinder-bed," formed 

 of a vast accumulation of shells of Ostrea distorta (fig. 369). In the 

 uppermost part of this bed Professor Forbes discovered the first echino- 

 derm (fig. 370) as yet known in the Purbeck series, a species of Ilemici- 

 daris, a genus characteristic of the Oolitic period, and scarcely, if at all, 

 distinguishable from a previously known oolitic species. It was accom- 



Fig. 369. 



Fig. 3T0. 



Ostrea distorta. 

 Cinder-bed, Middle Purbeck. 



Uemicidaris PurbecTcensis, E. Forbes. 

 Middle Purbeck. 



panied by a species of Perna. Below the Cinder-bed freshwater strata 

 are again seen, filled in many places with species of Cypris (fig. 371, 



Fig. 371. 



Cyprides from the Middle Purbecks. 



a. Cypris striato-punctata, E. Forbes, b. Cypris fascioulata, E. Forbes 



c. Cypris granulata, Sow. 



a, b, c), and with Valvata, Paludina, Planorbis, Limnaius, Physa 

 (fig. 372), and Cyclas, all different from any occurring higher in the 



