in the Dorsetshire Fur becks. 393 



bands of cherty stone, filled with these fossils in a beautiful i^tite of 

 preservation, occur, and among them are, for the first time, in the 

 oolitic series, Gyrogonites, the spore-capsules of Charae. Imme- 

 diately above these interesting bands (in which many remains of fish 

 were also found) is the great and conspicuous stratum, long familiar 

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

 vast accumulation of the shells of Ostrea distorta. On the summit 

 of this bed, the author discovered the first Echinoderm ever seen in 

 the Purbecks. It proved to be a species of Hemicidaris, a genus 

 characteristic of the oolitic period. It was accompanied by a species 

 of Perna. The cinder-bed is succeeded by limestones and shales, 

 partly of freshwater, partly of brackish-water origin. In these, the 

 same species of Cypris occur, which mark the shaly bands near the 

 chert below the cinder. 



Many fish, especially species of Lepidotus and Microdon radi- 

 atus are found in these sands ; and in the fine collection of Mr Wilcox 

 of Swanage, are the heads of two magnificent species of the reptile 

 MacrorynchuSy resembling, but distinct from that described by Von 

 Mayer, from this horizon in the Purbecks. Among the Mollusks, 

 a remarkable ribbed Melania, of the section Chilina, is found here. 

 After the deposition '^f these strata, there came another powerful 

 influx of the sea, introducing marine species, PectenSy Modioke^ 

 AviculeB, and Thracecp^ all undescribed forms. Brackish-water strata 

 full of Cyrena^ and traversed by bands abounding in CorhultR and 

 MelanicB are next in order : in them is a Protocardium, a large 

 species quite distinct from its representative species in the lower 

 portion of the Purbecks. Limestones with Cyprides, Turtles and 

 fish, crown these brackish-water bands, and are specifically con- 

 nected with the beds of the middle Purbecks below them. 



Lastly, a third series of freshwater strata commences with a new 

 series, — Cyprides, Paludince, Physa, Limneus, Planorbis, Valvata, 

 CycladeSj and Unio, and new forms of fish. These continue until 

 they merge into the base of the Hastings sands, and the Purbeck 

 series is completed. The total thickness of all the Purbecks at 

 Meup's Bay is about 155 feet. Of this, one-half is occupied by 

 the lower portion of the series, and the remainder is divided between 

 the middle and upper portions, the former being rather the more 

 extensive. 



It is very remarkable, that whilst we can strictly divide the Pur- 

 becks into upper, middle, and lower, each marked by a peculiar 

 assemblage of organic remains, the lines of demarcation between 

 these sections are not lines of disturbance, nor indicated by striking 

 physical characters or mineral changes. The features 'which attract 

 the eye in the Purbecks, such as the dirt-beds, the dislocated strata 

 at Lul worth, and the cinder-bed, do not indicate any breaks in the 

 distribution of organized beings. The causes which led to a com- 

 plete change of life three times during the deposition of the fresh- 



