INOCERAMUS. 2G7 



Noil 1875. Inoceeamus concentricus, A. J. Jukes-Browne. Quart, Jouni. Gcol. Soc, 



vol. xx.\i, p. 21)9. 



Desciiplinii. — Slicll iniiiv or less ovaic, much liii^-licr tliaii long, vei'v iii('(|iii- 

 lateral and ineqnivalve. 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 uml)ones. 

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

 nai'ruw, pointed, terminal uniljo 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 nuTuerous 

 ivgular 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. — /. concctifriciM has been compared l)y Wolleniaun with 7. Val-oIiH, 

 Schli'iter,^ but the prominent umbo and other characters readily distinguish it from 

 that species. See also I. cuncentricus var. snbsHlcatuf, and I. snlcatn.<! (below). 



liemarJcs. — Whilst the left valve in the specimens from the Blackdown Greensand 

 agrees closel}- with that of specimens found in the Gault, the right valve in many 

 (but not all) cases is relatively moi-e convex, especially between the umbo and the 

 postero-veiitral extremity, and the ribs are more sharply cui'ved. A similar 

 modification is seen in the examples from the Red Limestone of Hunstanton. 'J'liere 

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

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

 different from those Avliich prevailed where the Gault was deposited. 



/. r/ri/plicCuichs, Sowerby, was founded on an internal cast of /. i-oiirnitrirns 

 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 Candjridge Greensand which have been recorded as 

 I. coiiccnfricKs ai)j)ear to l)elong to 1. aiir/licii-s (p. 2(1 1). 



Tijpes. — The type, which came from the Gaidt of Folkestone, cannot now be 

 found. The specimens from the same locality figured by Sowerby and by ]\hintel!, 

 and the type of /. ijnjphxo'ules from the Upper Greensand near hyiiu' Rt-'gis, are in 

 the Hiitish ^luseum. 



Didiiliatinii. — In all zones of the (iault of Folkestone. (!ault of Aylesford, 



1 'Mouatsber. d. k. piviiss. Aka.l. Wissenscli., Birliu' (18(30), p. 345. Schlutor, ' Piiliuonto- 

 iji-.ipliicii,' vol. .\xiv (1877;, p. 255. Wollomaim, *• Bivalv. u. Gastiop. J. iiorddeutSL-h. Gaults" 

 (' Jaliil). d. K. prciiss. geol. Laudesaust.,' vol. .\.\vii, 1900), p. 272, pi. vi, fig. 9. 



