NEOCOMIAN. 145 



them are the short stems of Cycads, which resem- 

 ble the living Cycas revoluta. 



The Lower Purbeck also contains a multitude 

 of insect remains, on several different horizons. 



chiefly in cream-coloured marl, and 



include the wing Cases and bodies ^i beetles, 

 , and other insects. 



A terrestrial surface at the base of the Middle 



Purbeck is evidenced by many jaws, and a few 

 other remains of small mammals. They appear 

 to be little insectivorous marsupials, such as 



\lacotherium^ ami the type like the 

 kang at, which is named Plagiauiax. 



The Middle Purbeck includes some marine 

 layers with cockle shells, oysters and pectens, 

 with occasional ripple-marked sandstone. The 

 marine beds alternate with the fresh-water beds, 

 in which the common shells which live in cxist- 



ponds, Paiudina^ Planorbis % Physa % etc., appear 



for the first time. 



The Upper Purbeck beds are interesting, first, 



for yielding the grey, Paludina marble, anciently 



used for decorative carving and monuments in 



interior of churches; and, secondly, for the 



number of its fresh-waler tortoises, named Plcuro- 



rum, which differ from their living allies in 

 having an extra pan* of bones, stretching over 

 the middle of the breast-plate, known as the 

 plastron. 



In Swanage Bay the overlying Wealden beds 

 differ from the Purbeck beds in being sands and 

 clays. In the boring near Battle in Sussex, the 

 Purbeck beds, formerly known as the Ashburn- 

 ham beds, contain some important layers of gyp- 

 sum, but no other calcareous deposits; and are 

 like the Wealden beds in mineral character. 

 10 



