DORSETSHIRE PUSBECKS. 399 



containing Paludina, PJiysa, Lymncea, Planorlis, Valvata, 

 Cyclas, and Unio, with the shields of cyprides, and remains 

 of fishes. 



Middle Series. Consist of 30 feet of strata, deposited 

 under changing conditions ; the uppermost beds are of 

 fresh-water limestone, with cyprides, turtles, and fishes of 

 different species from those found in the upper series. 

 Below the limestone are brackish water beds full of Cyrena, 

 and traversed by seams of Corbulcs and Melanite. These 

 are based on a marine deposit containing Pecten, Modiola, 

 Avicula, Thracia, of new species. Below this, again, come 

 limestones and shales, partly of brackish and partly of fresh- 

 water origin, containing fishes of the genera Lepidotus and 

 Microdon, and a reptile called Jlfacrorhyncus. Immediately 

 below is the " Cinder-bed," about 12 feet in thickness, 

 formed of a vast accumulation of Ostrea distorta, Perna, and 

 a new species of Hemicidaris, an urchin which is chiefly 

 confined to the oolites. Below the " Cinder-bed," fresh- 

 water strata again occur in many places filled with species 

 of Cypris, Valvata, Paludina, Planorbig, Lymncea, Physa, 

 and Cyclas, with seeds of Charge, distinct from those found 

 in the beds above : beneath these fluviatile beds thin bands 

 of greenish shales, with marine shells and impressions of 

 leaves like those of a large Zostera, form the basement of 

 the middle division. 



Lower Series. Beneath the marine bed just described, 

 fresh-water marls occur, containing Cypris, Valvata, and 

 Lymncsa, of species different from those of the middle 

 division ; this is the beginning of the lower series, which is 

 about 80 feet in thickness. Below the marls are 30 feet of 

 brackish water beds, with Serpula, Rissoa, and Cardium, 

 together with Cypris; the "dirt-bed," with the roots and 

 stools of Cycadece, underlies these marls, resting upon the 

 lowest fresh-water limestone, a rock about 8 feet thick, 

 ^containing Cyclades, Valvata, and Lymncea, of the same 

 species as those of the uppermost part of the lower series. 

 This limestone rock rests upon the superior beds of the 

 Portland stone, which is purely marine, and between which 

 and the Purbecks there is no passage. 



The following table * will enable the reader to understand 



* Sir C. Lyell's Elements, Third Edition, page 235. 



