INOCERAMUS. 2G7 



Non 1875. Inoceeamus concentkicus, A. J. Juices-Browne. Quart. Journ. Geol. Soc, 



vol. xxxi, p. 299. 



DescrijAlon. — Shell more or less ovate, niucli higher than long, very inequi- 

 lateral and inequivalve. Anterior part of valves flattened or concave, often more 

 or less nearly perpendicular to the plane between the valves. Posterior part of 

 the valves somewhat expanded and less convex than the part below the umbones. 

 Postero- dorsal part forming a small wing. Left valve very convex, with a high, 

 nai-row, pointed, terminal umbo which is considerably incurved and bends forwards. 

 Right valve considerably less convex than the left valve, with a small, terminal 

 umbo curved forwards but only slightly inwards. Hinge-line less than half the 

 height of the shell. 



When the shell is perfect the surface is nearly smooth except for numerous 

 regular growth-rings. When the outer layer of the shell is wanting concentric 

 undulations or ribs having an unsymmetrical curvature are seen, and are separated 

 by concave furrows. 



Affinities. — I. concevtririis has been compared by Wollemann with I. Eicaldi, 

 Schliiter,^ but the prominent umbo and other characters readily distinguish it from 

 that species. See also f. concentvicus var. sithsulcatiis, and I. sulcatum (below). 



Remarks. — AVhilst the left valve in the specimens from the Blackdown Greensand 

 agrees closely with that of specimens found in the Gault, the right valve in many 

 (but not all) cases is relatively more convex, especially between the umbo and the 

 postero-ventral extremity, and the ribs are more sharply curved. A similar 

 modification is seen in the examples from the Red Limestone of Hunstanton. There 

 seems no reason for regarding the specimens with these characters as forming 

 more than a local variety dependent on the fact that they lived under conditions 

 different from those which prevailed where the Gault was deposited. 



/. grypliseuides, Sowerby, was founded on an internal cast of I. conceiitricns 

 from the Upper Greensand ; in such casts the ribs are more prominent than on 

 the surface of the shell itself. 



All the examples from the Cambridge Greensand which have been recorded as 

 I. coitceutricus appear to belong to I. anglicus (p. 264). 



Tijpcs. — The type, which came from the Gault of Folkestone, cannot now be 

 found. The specimens from the same locality figured by Sowerby and by Mantell, 

 and the type of I. grijphseoides from the Upper Greensand near Lyme Regis, are in 

 the British Museum. 



Distribution. — In all zones of the Gault of Folkestone. Gault of Aylesford, 



1 ' Monatsber. d. k. preuss. Akad. Wisseusch., Berlin' (1860), p. 345. Schliiter, ' Palseonto- 

 graphica,' vol. xxiv (1877), p. 255. Wollemann, " Bivalv. u. Gastvop. d. norddeutsch. Gaults" 

 (' Jahrb. d. k. preuss. geol. Laudesanst.,' vol. xxvii, 1906), p. 272, pi. vi, fig. 9. 



