268 CRETACEOUS LAMELLIBRANCHIA. 



Dunton Green (Kent), Black Ven, and Okeford Fitzpaine. Upper Greensand 

 (zone of ScJilo'iihdcJiia ro.sfnita) of Blackdown, Sidmouth, Lyme Regis, and Potterne 

 near Devizes. Limestone in the Ganlt of West Bilney, Roy don, and Grimston 

 (West Norfolk). Red Limestone of Hunstanton. 



Inoceramus coNCENTRicns var. suBSULCATUS, WiJtshirr, 18G0. Plate XLVIT, figs. 



3—] 4. 



1853. Inoceramus svi.catvs, F. J. Plctet and W. Monx. Moll. Foss. Gres verts 



de Geutve. p. 499, pi. 

 xlii, figs. 1 d, e, f. 



1869. — s^BSVLCArvs, T. Wiltshire. Quart. Jouni. Geol. Soc, vol. xxv, 



pp. 188, 190. 



1879. - — F. G. H. Price. The Gault, p. 54. 



In the lowest zone of the Upper Ganlt there occur forms of Inocerai)ii(s which 

 differ from /. concentricus only in the fact that radial plications are more or less 

 extensively developed. In these forms every stage can be traced between I. con- 

 centricus without plications into forms in which the plications ai"e almost as 

 complete as in I. sulcatus. A specimen, which differs from /. concentric ns only in 

 the occurrence of one shallow plication, is shown in fig. 3 (Plate XLVII) ; others 

 are similar, but possess two or three plications (Plate XLVII, figs. 4, 5). In most 

 cases the early part of the shell agrees entirely with /. concentricus ; but the 

 duration of this stage varies considerably in different individuals, and sooner or 

 later either a few or many plications are developed. When the plications are 

 numerous and start from near the umbo the shell closely resembles /. sulcatns, 

 but may usually be distinguished by the rounded (instead of angular) form of 

 the plications, and by the presence of concentric ribs. For these forms, which 

 characterise a definite horizon in the Gault, it will be convenient to adopt as a 

 varietal designation the name suhsnicafiis, given by Wiltshire in 1869. 



From a study of many forms of this variety it seems evident that I. suJcafus 

 has been derived from 7. concentricus by the gradual development of plications, and 

 this view is in accordance with stratigraphical distribution, since I. concentricus is 

 the earlier of the tw^o sj^ecies, being found throughout the Gault, wdiilst /. sulcatus 

 occurs in the upper part of the Gault only. 



If I. sulcatus has been derived from /. concentricus, then it is clearly more 

 nearly related to that than to any other species of Inoceramus, and cannot therefore 

 be placed in a separate genus or sub-genus (Actiiioceramus), as was proposed by 

 Meek.^ Further, in the early stages of the varietj suhstdcatus the shell is a typical 



1 'Check List Cret. Foss. N. Amer.' ('Sinilhson. Miscell. Coll.,' No. 177, 1864), p. 32. Stoliczka, 

 'Cret. Fauna S. India,' vol. iii (1871), p. 393. Meek and Hayden, ' Invert. Cret. Tert. Foss. U. 

 Missouri' (1876), p. 39. 



