§ 24. ECHINITES OF THE CHALK. 329 



great variety of species, which are arranged in numerous 

 sub-genera, for the convenience of study ; but I can notice 

 only a few of the common forms. 



The helmet-shaped echinites (Lign. 62, Jig. 5) are ex- 

 tremely abundant, and in some localities occur in shoals, 

 and in every gradation from the young to the adult state. 

 Siliceous casts, formed by the decomposition and removal of 

 the shell from the flint with which it was filled, are common 

 in gravel and on ploughed lands. The cordiform variety 

 (Jig. 7) is very abundant, and gives rise to the heart-shaped 

 flints of our gravel-pits. The elliptical species (Jig, 3) is 

 common in the greensand. The turbinated echini ( Cidares), 

 are beautifully embossed with papillae : a small species 

 (Jig. 1) is frequent in the chalk and flints of Kent ; the 

 larger varieties possess tubercles (Jig, 6), surrounded by 

 elegant margins, and are otherwise richly ornamented. 

 Some spines are slender, and covered with asperities (Jig. 2) ; 

 others almost smooth (fig, 9), and club-shaped (fig, 4) ; it 

 is not often that spines are found in contact with the 

 shell (fig, 8), this however, is occasionally the case, and 

 from the chalk at Gravesend, and other localities in Kent, 

 splendid examples have been discovered ; some with groups 

 of large spines attached to the tubercles, as in the living 

 state. 



24. Shells of the chalk. — The bivalve shells, or 

 conchifera of the chalk, are very numerous ; of one genus 

 alone (Terebratula), above fifty species are enumerated. 

 Oysters, scallops, areas, tellens, and other familiar marine 

 shells abound, but the species differ from the recent. With 

 these known genera are many which, so far as our present 

 knowledge of the inhabitants of the deep extends, are 

 extinct. Two or three species of Cirrus are not unusual in 

 the English white chalk ; but the univalves are few ; and the 

 only specimen of a large simple spiral univalve with which 

 I am acquainted is a species of Dolium, of which several 



