xvi. POTTON BONE BED. 429 



of animals and different parts of the bodies. Among these vertebrae 

 are frequent, and to them iguanodon, megalosaurus, plesiosaurus, 

 and ichthyosaurus have contributed largely. There are very 

 many teeth, some of iguanodon, some of megalosaurian, others of 

 crocodilian affinity; besides these are defensive spines of hybodus 

 and asteracanthus, teeth of sphserodus and pycnodus. Separate 

 or surrounded by indurated clay are plenty of ammonites, rarely 

 in a state to be identified for species, a few belemnites, a few 

 gasteropods, a few oysters. Rarely a fragment of a cycadaceous 

 stem rewards a search on the enormous heaps of the excavated 

 gravel. 



The gravel is largely dug for its phosphatic wealth; bones, teeth, 

 and nodules generally, contain more or less of this precious matter, 

 and they are sorted according to their value, and, for a different 

 reason, according to the size. 



Thus a favourable opportunity occurs for collecting on a great 

 scale a large variety of fossils; but they are generally found to 

 have been much injured by rolling and mutual attrition in water. 



In considering the history of this deposit, the geological age 

 and circumstances of accumulation are the most important objects 

 to be attained; and the most valuable evidence is furnished by 

 the organic remains : for these, lying as they were placed by 

 the agitated water, give more than suggestions of the processes 

 which preceded their accumulation. 



The geological date of the deposit may be safely taken as 

 corresponding to the period between the Portland oolite and the 

 gault that period of partly marine, partly estuarine, and partly 

 fresh-water strata, known as Purbeck, Wealden, and part of the 

 lower greensand, approximately agreeing with the ' Neocomian ' 

 ages. 



The fossil remains, when considered in relation to their proved 

 or assumed places in geological time, give the following results 

 as a first approximation. Very few specimens can be confidently 

 declared to be known as resident in, or peculiar to, or characteristic 

 of the lower greensand; some are found plentifully, even charac- 

 teristically, though not exclusively, in the Wealden ; one or two 

 are on record from the Purbeck beds and the Portland rock ; 

 several from the Kimmeridge, and one or two from the Oxford 

 clay. These statements are made on the evidence of specimens 



