$4 HANDBOOK OF FOSSILS. 



are next coming to with the Wealden just passed. They contain 

 numerous fresh- water shells Paludina t Physa^ Limna:a t etc., 

 with the microscopic valves of the little fresh-water crustacean 

 Cyan's, whose descendants are abundant in the rivers and lakes of 

 to-day. An oyster occurs in the " Cinder Bed" and Plant remains 

 in the "Dirt Beds." But the Purbecks are best known for the 

 numerous remains of small mammals (Plagianlax) allied to the 

 kangaroo rat, at present living in Australia. 



b. The Portland Stone aitd Sand, which come next in order, 

 are largely quarried in the island whence they take their name. 

 The quarrymen point out fossils in the stone, which they call 

 " Horses'-heads " and "Portland screws." The former is the 

 cast of a Trigonia shell ; the latter, that of a tall spired univalve 

 (Ceritkiutfi). 



In Wiltshire, a coral (Isastrea ollonga} is found in the sandy 

 beds, the original calcareous matter of which has been replaced 

 by silex. 



c. KimmeriJge Clay. This, by the pressure of the rocks subse- 

 quently deposited on it, has in greater part been hardened, and 

 possesses a tendency to split in thin layers, and hence is termed 

 by geologists a shale. It is seen at various points between Kim 

 meridge on the Dorsetshire coast and the Vale of Pickering in 

 Yorkshire, and forms broad valleys. The principal fossils in it 

 are Ammonites, a triangular-shaped oyster (Ostrea deltoidca), and 

 one resembling a comma (Exogyra virguld], 



Middle Oolites. 



a. The Coral Rag, or Coralline Oolite, comprises a most variable 

 set of beds, but principally a series of limestone, with fossil 

 corals still in the position in which they grew, and resembling 

 in form the reef-building corals of the Pacific. They rest on 



b. Oxford Clay, a dark blue or slate-coloured clay without any 

 corals, but containing a great many Ammonites and Belemnites. 

 The Kelloivay Rock, a sandy limestone at the base of the Oxford 

 Clay, is well developed in Yorkshire, and furnishes amongst 

 other fossils a large belemnite and an oyster (Gryphaa dilatatd). 



Lower Oolites. 



a. Cornbrask, a very shelly deposit of pale-coloured earthy, and 

 rubbly or sometimes compact limestone with plenty of fossils. 

 The commonest are Brachiopods, Limas, oysters (Ostrea Mar shit), 

 Pholadomyas and Ammonites. It is best seen in Dorsetshire, 

 Somersetshire, and near Scarborough in Yorkshire. 



b. Forest Marble and Bradford Clay. The former is an ex- 

 ceedingly shelly limestone, often splitting into thin slabs. On the 

 surfaces of some of the beds may be seen the ripple marks the 



