398 GEOLOGY OF THE ISLE OF PORTLAND. 



has named Mantellia. There are two species ; one is 

 short, and of a spheroidal form (31. nidiformis) ; the 

 other is longer, and sub-cylindrical (M. cylindrical) : 

 the specimens of the former species are called 

 " Crows -nests' by the workmen, who believe these 

 plants to be bird's nests, originally built by crows 

 in the fossil trees, which have become petrified. 

 The largest specimens are about two feet high, and 

 three feet in circumference.* 



The Pine trees and the Mantelliae are completely 

 silicified, and their internal structure is beauti- 

 fully preserved in many examples ; the wood, 

 when microscopally examined, displays the Arau- 

 carian structure (see p. 283). A cone has been 

 found in the dirt-bed, which Dr. Brown considers 

 to be closely related to the fruit of the Norfolk 

 Island Pine (Araucctria excelsd).^ The Portland 

 and Isle of Wight fossil coniferae appear to belong 

 to one species. 



Above the dirt-bed are layers of finely lami- 

 nated cream-coloured limestone, the total thick- 

 ness of which is about ten feet ; in these beds I 

 have not discovered any traces of organic remains, 



* These fossil Cycadeae are admirably illustrated by Dr. Buckland in Geol. 

 Trans, vol. ii. new series ; and also in his Bridgewater Essay. A fine suite 

 of specimens, formerly in my collection, is exhibited in the British Museum. 



t Dr. Pitton. 





