I 24. SHELLS OF THE CHALK. 331 



The sub-globular terebratuke, both the common and the 

 striated varieties, are very abundant.* Another bivalve 

 equally numerous, is an elegant shell, having one valve 

 covered with long spines (Lign. 63, fig. 8), the Plagiostoma 

 spinosum.^ A bivalve with a fibrous structure (Tnoceramus, 

 Lign. 63, fig. 4), very brittle, and having a crenulated 

 hinge of a peculiar construction, presents numerous species ; 

 some of which are small and delicately striated, while others 

 are two feet in diameter, and deeply furrowed.J The sub- 

 stance of these shells closely resembles in structure that of 

 the recent pinnce; and from its fragility, fragments are very 

 common in chalk, flint, and even in pyrites. § The Gait 

 contains two species of this genus, which have been found 

 in almost every locality, and appear to be restricted to this 

 division of the chalk ; they are the Inoceramus concentricus 

 {Lign. 63, fig. 1), and I. sulcatus (fig. 3) ; a hybrid occurs 

 in the Folkstone beds, partaking of the characters of both. 

 I have discovered a species of Spherulite (S. Mortoni) in 

 the chalk near Lewes, in Sussex ; but Hippu?'ites, so com- 

 mon in the cretaceous strata of the Continent, have not been 

 noticed. || 



The shells of the Marl, Gait, and Greensand, amount to 

 many hundred species : those of the whetstone pits of 

 Blackdown, in Devonshire, being completely silicified, and 

 changed into silex, jasper, or chalcedony.^" 



* Medals of Creation, vol. i. p. 379. 



f This shell was formerly supposed to be peculiar to the chalk 

 strata, but specimens have been discovered by Mr. Pratt in eocene 

 deposits. Medals of Creation, vol. i. p. 390. 



♦ Ibid. p. 393. § Ibid. p. 396. 



|| Ibid. p. 428. No true Hippurites have been found in the English 

 chalk ; the shells of the family Kudistes hitherto met with have the 

 characters assigned to the genus Spherulites. 



^[ The shells of the cretaceous formation are most fully described 

 and figured in M. D'Orbigny's beautiful work on the fossils of France 

 — Palaeontologie Francaise. 



