410 CRETACEOUS LAMELLIBRANCHIA. 



Upper G-reensand, and of which typical examples were figured by Sowerby in 1829, 

 have a small surface of attachment, and have been known in this country as Exogyra 

 conica (figs. 220 — 225). A similar form, but of larger size, is common in the 

 Chalk Marl (figs. 228, 229), and the Glauconitic Sandstone of Antrim; no line can 

 be drawn between this and the smaller form of E. conica, with which it has been 

 united by most authors (e. g. d'Orbigny, Morris, Coquand, Bayle, Peron) ; examples 

 indistinguishable from E. conica of the Upper Greensand are associated with the 

 larger form in Antrim, and on the other hand, some specimens found in the Upper 

 Greensand of the south of England are of larger size than the majority of examples 

 and cannot be separated from the larger form of E. conica; but nevertheless the 

 larger form appears generally to mark a somewhat higher horizon than the 

 smaller {conica, Sowerby) form. 



E. haliotoidea, Sowerby, from the Upper Greensand (fig. 240), is a small form 

 of E. conica with a large surface of attachment, and is connected by intermediate 

 forms with examples having only a small attached surface. Similarly E. Rauliniana, 

 d'Orbigny (fig. 236 — 239), from the Gault, Upper Greensand, and Lower Chalk, 

 is a large form of E. conica with a large surface of attachment and is linked by a 

 series of gradations (figs. 230 — 235) to the type with only a small attached 

 surface. Peron 1 has already shown that the large specimens from the Cenomanian 

 of France figured as E. ludiotoidea by d'Orbigny and Coquand are only forms of 

 E. conica modified by having a large surface of attachment; they occur in the 

 same beds as undoubted examples of E. conica. In some specimens with a large 

 attached surface the height of the shell is greater than usual, but this, as pointed 

 out above, is due to the shell being fixed mainly or entirely by the posterior 

 slope behind the carina; in such cases the free marginal part forms a sharp angle 

 with the attached surface. Morris and Jukes-Browne united E. haliotoidea and 

 E. Rauliniana, and Pictet and Campiche recognised the very close relationship 

 of these two forms. 



Examples from horizons above the Cenomanian have been referred to E. halio- 

 toidea by some authors, 1 notably Hennig, who has compared the Senonian forms 

 of Sweden with specimens from the Cenomanian and Gault. Without the oppor- 

 tunity of examining the Senonian forms I am unable to express any opinion as to 



1 'Compte Rend. Assoc, franc. Avanc. Sci.,' xxxvi (1908), pt. 2, p. 312. 



2 Chama haliotoidea, Nilsson, Petrific. Suecana (1827), p. 28, pi. viii, fig. 3 ; Hisinger, ' Letbaea 

 Suecica' (1837), p. 62, pi. xix, fig. 3. Exogyra haliotoidea, Eeuss, ' Die Verstein. d. bohm. Kreideformat.,' 

 pt. 2 (1846), p. 44, pi. xxvii, figs. 5, 9, 10, pi. xxxi, figs. 9 — 10; Muller, 'Petrefact. Aachen. Kreidef., 

 pt. 1 (1847), p. 42 ; Schroder, 'Zeitscbr. d. deutsch. geol. Gesellsch.,' vol. xxxiv (1882), p. 260, pi. xv, 



5; Griepenkerl, 'Pala^ont. Abhandl.,' vol. iv (1889), p. 36; Hennig, 'Eevis. Lainellibr. i Nilsson's 

 ' Petrific. Suecana ' (1897), p. 19, pi. i, fig. 20, pi. ii, figs. 3, 4 ; Eutot, ' Bull. Soc. Beige Geol. Pal. et 

 Hydrol.,' vol. x (1897), p. 27, fig. 11. 



