2G8 CRETACEOUS LAMELLIBRANCHIA. 



Dunton Green (Kent), Black Yen, and Okeford Fitzpaine. Upper Greenland 

 (zone of Schlcenhachia rosirata) o^ Blackdown, Sidmoutli, Lyme Regis, and Potterne 

 near Devizes. Limestone in tlie Gault of AVe.st Bilney, Roydon, and Grimston 

 (West Norfolk). Red Limestone of Hunstanton. 



Inoce1!.\mu.s coNCKNTiiicrs var. sriiSLTi.f'ATi'S, WilUhiri', 1869. Plate XfiVII, figs. 



3—14. 



1853. Inoceeamus svt.cA-rvs, F. J. Pictcf and W. Ronx. Mull. Fos.s. Grcs verts 



de Geneve, p. 499, pi. 

 xlii, figs. 1 d, e, /. 



1869. — svbsvIjCatvs, T. Wllfyh ire. Quart. Jouru. Geol. Soc., vol. xxv, 



pp. 188. 190. 



1879. — — F. Cf. H. Pm-r. 'Ihe Gault, p. 54. 



In the lowe.st zone of the U])per (iuult there occur tornis of Lioccrainns which 

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

 extensively developed. Li these forms every stage can l)e traced l)etween /. con- 

 centricus without plications into forms in which the plications are almost as 

 complete as in /. snlrnfiis. A specimen, which differs from I. concent ricns only in 

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

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

 cases the early ])art of the shell agrees entireh' Avith I. 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 I. sulcntnx, 

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

 the plications, and by the presence of concenti'ic ribs. For these forms, which 

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

 varietal designation the name suhsnlcatns, given by Wiltshire in 18(59. 



From a study of many forms of this A'ariety it seems evident that /. snlmtus 

 lias been derived from /. concrntricns by the grailual di'velopment of plications, and 

 this view is in accordance with stratigraphical distribution, since /. concent ri ens is 

 the earlier of the two species, being found throughout the Gault, whilst 7. snJcatns 

 occurs in the upper })art of the Gault only. 



If 7. snlcatns has been dei'ived from I. coiicentricn.s, then it is clearly more 



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



1)0 placed in a separate genus or suli-genus [Aetinoeennnnx), as was proposed by 



Meek.' l"'urthef, in the early stages of the \i\v\vty sn1>snlcatns the shell is a typical 



1 ' Clieek List Ciet. Foss. N. Amer.' (' Suiitliscu. Miscell. Coll.,' No. 17", 1864), p. 32. Stoliezka, 

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

 Missouri' (1876), p. 39. 



