324 
ELEMENTS OF GEOLOGY. 
are very abundant and characteristic. (See Figure 299, 
a, 5, a) 
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. 
a. Cypris gibbosa, E. Forbes, b. Cypris tuberculata. E. Forbes, c. Cypris leguminella^ 
E. Forbes. 
Middle Purbeck, —Next in succession is the Middle Pur- 
beck, 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 Lepi- 
dotus and Microdon radiatus,^ are found, and a crocodilian 
reptile named Macrorhynchus, Among the mollusks, a re¬ 
markable ribbed Melania,^ of the section Chilina^ occurs. 
Fig. 301. 
Ostrea distorta, Sow. Cinder- Hemicidaris PurbecJcensis, E. Forbes, 
bed, Middle Purbeck. Middle Purbeck. 
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 
