151 



Ananchytes, Lam. (helmet-shaped), is found here only : no 

 individual has been noticed as having been found in any of 

 the preceding subjacent formations; nor are any found in 

 the posterior formations or among the echinidae of the 

 present seas. Specimens of the galerites, Lam. frequently 

 occur here ; but, as has been observed respecting clypeus 

 and ananchytes, this genus is only known in its mineralized 

 remains. 



The spatangites are also frequently found in the chalk. 



Numerous specimens of the genus cidaris are also found 

 in the chalk, and particularly in the Kentish chalk pits, 

 where several different species are discovered, and not 

 uncommonly those, which, from their being highly orna- 

 mented, may be termed diadems, many of which have their 

 tubercles perforated in the centre, and have also the bases 

 of their papillary projections, on which the spines articulate, 

 ornamented by regular crenulations. 



Siliceous casts of the different species which exist in a 

 mineralized state are frequently found in the gravel derived 

 from the chalk ; particularly of the smaller gcderit<^, and 

 which, from their general diffusion, and from the irregular 

 variation of their forms, not allowing their separation into 

 distinct species, have been spoken of as echinitce vulgares. 



Shells. — Recent shells are divided into univalves, bivalves, 

 and multivalves ; the univalves being also divided into those 

 which have one or more chambers, being unilocular or mul- 

 tilocular. But another section is necessary for the re- 

 ception of those extinct shells of the former world which 

 have been found in the earlier strata, and which, though 

 bearing the outward appearance of bivalve shells, are di- 

 vided by septa into several chambers. Such are productus 

 of Sowerby, first noticed by Martin ; and pentamerus dis- 

 covered by Mr. Aikin in the mountain limestone of Shrop- 

 shire, the nature of which shell has been so successfully 



