334 CRETACEOUS LAMELLTBRANCHJA. 



of I. involutus by Barrois. /. Koeneni, Midler, 1 is probably a variety of 

 /. involutus in which the right valve is more convex and its umbo more prominent 

 than usual. 



The right valve of the specimen figured by d'Orbigny (184G, pi. ccccxii, 

 figs. 1, 2) as I. Lamarclci is an example of /. involutus; but the left valve (fig. 3), 

 if it belongs to the same individual, is probably incorrectly drawn. 



Remarks. — Examples of this species often reach a large size, and in such cases 

 the hinge (Fig. 94) may attain a considerable thickness, but portions of it are 

 not often found separately. In old specimens the marginal part of the right 

 valve grows obliquely or almost at right angles to the earlier part, and the folds 

 become indistinct or disappear altogether ; and in the right valve this marginal 

 growth sometimes occurs along the hinge, owing no doubt to the increase in size of 

 the left valve in which it then fits like an operculum (Fig. 94). The length varies 

 in proportion to the height, so that in some forms the right valve becomes nearly 

 circular. 



Types. — The type (Fig. 88) is in the British Museum, No. 432G8 ; its locality is 

 unknown. The specimen figured in Dixon's ' Geology of Sussex ' (Fig. 89) is also 

 in the British Museum, No. L83 ; it is a flint cast and came from the Upper Chalk 

 of Charing. 



Distribution . — 7". involutus is found in the zone of Micraster cor-testudinarium 

 and the lower part of the zone of M. cor-anguinum, being particularly common at 

 the latter horizon. 2 



Zone of ill. cor-testudinarium : Chatham, Dover, and Seaford, Sussex. 



Zone of M. cor-anguinum : Winchester, Quidhampton, Mapledurham, Thanet, 

 St. Margaret's, Guilford Colliery (Coldred near Dover), the Sussex coast, Lewes, 

 Haling pit (South Croydon), Strood, New Brompton (Chatham), Gravesend, 

 Harefield near Rickmansworth, Bury St. Edmunds, Saham Toney, Thetford, 

 Brancaster, and other places in Norfolk. Between the zones of M. cor- 

 testudinarium and M. cor-anguinum near Beverley, Yorkshire. Senonian of 

 Haldon. 



Inoceramus cordiformis, Soiverby, 1823. Plate LIII, fig. 8. Plate LIV, figs. 2—4. 



1823. Inoceramus cordiformis, /. da C. Soiverby. Min. Couch., vol. v, p. 61. 



pi. ccccxl. 

 183G. A. Goldfuss. Petref. Germ., vol. ii, p. 113, 



pi. ex, fig. 6b (not 6a). 



' Jalirb. d. k. preuss. geol. Landesanst.' fur 1887 (1888), p. 412, pi. xvii, fig. 1. 

 2 This species has beeu recorded by Griffith and Brydone from the Uintacrinus band of the 

 Marsupites zone of Bopley, Hampshire ; and by Barrois from the zone of Actinocamax quadrahis of 

 Newhaven. 



