D inococl ilea ing ens frovi the Wealden Beds, Hastings. 243 



water-worn cavities subseq^uently filled up. A minor difference 

 between them and the true concretions in the same deposit, but a 

 noteworthy one, is that v.'^hile the latter either exfoliate, or split 

 in any direction, the spirals rarely exhibit exfoliation and usually 

 fracture in a plane at right angles to their axis, generally directly 

 through the whorl, but often in a plane more or less coincident with 

 the line of suture between the whorls. The two systems sometimes 

 occur in close proximity, wedge-shaped pieces resulting. Finally, 

 the twist of these bodies, as will be seen later, conforms to the law 

 of the logarithmic spiral, which Canon Moseley and others have 

 shown to be characteristic of the moUuscan shell, ^ whilst no known 

 concretion does so. 



The conclusion seems, therefore, inevitable that these spiral 

 bodies represent the internal casts of examples of a huge univalve, 

 niolluscan shell, some 6 feet and more in length, whose existence 

 has hitherto been unsuspected. 



Now the only molluscan shell comparable to this as a concrete 

 whole in size is that of the Carboniferous Aetinoceras giganteum 

 (Sby.), the length of which was coiuputed by its author to have 

 sometimes exceeded 8 feet, an estimate, however, which possibly 

 did not err on the side of moderation. Recollection of this, and the 

 large size of the protoconch in the new find, naturally raised 

 the question whether it also represented a Cephalopod shell of the 

 Turrilite or Cochloceras descrijition. The general facies and the 

 total absence of any trace of septal sutures preclude this idea, quite 

 apart from the difficulty of reconciling the presence of a marine 

 animal in a deposit which only contains fresh- or brackish-water 

 forms. Nor, for similar reasons, as well as on account of bulk, 

 should affinity be sought with some terrestrial form, although the 

 elongate cylindrical shape and many whorls vividly recall Megaspira. 

 The recent fresh- or brackish-water mollusca that seem to come 

 nearest to the form in question belong to the Tiarida? {oli'n Melaniidas), 

 or such exceptional Cerithiidae as are known to penetrate into 

 brackish water, notably the Potamidinse, of which a possible repre- 

 sentative has already been recorded from the Wealden.^ 



^ Moselejr (Rev. H.), " On the Geometrical Forms of Turbinated and Discoidal 

 Shells " : Phil. Trans., 1838, pp. 351-370. For an able summary of the con- 

 clusions by Moseley and others, see Professor D'Arcy W. Thompson's Growth 

 and Form, chap, xi, pp. 493-.586. 



- The fossil in question is a cerithoid-like shell about 1^ in. in length, that has 

 been successively referred to : — 



Muricites strombiformis, Schlotheim, Petref., i, 1820, p. 144. 

 Melanopsis 1 tricarinata, J. Sowerby (Ann. Phi!., n.s. viii, 1824, p. 376, as 

 Melania tricarinata [n. nud.] and) Trans. Geol. 

 Soc, Ser. ii, vol. iv, 1836, p. 346, pi. xxii, f. 4. 

 Potamides carbonarius, A. Roemer, Nordd. Oolith., 1836, p. 141, pi. xi, f. 17. 

 Pleuroceras strombiforme, Schloth. : Sandberger, Land- u. Siissw. Conch. 

 Vorwelt, p. 55, 1870, pi. ii, f. 11 a-g. 



The genus Muricites, fomided by J. Qesner in 1752 {Tract. Petr., p. 34), is 

 pre-Linnean, and though the name appears again in the second edition of his 

 book in 1758 (p. 56), that does not by present ruling render the name valid. 



