44 Reports and Proceedings — Geological Society of London. 



I. cordiformis, Sow. (zone of M. cor-testudinarium to Uintacrinus 

 Band), has been derived from a varietj- of /. lamarclci, with inflated 

 and nearly equal valves, by the development of one or two radial sulci 

 and a relatively longer hinge. 



/. cosiellatus, Woods ' (Chalk Eock), appears to have come from an 

 early form of /. lamarchi by the axis of growth becoming more oblique 

 to the hinge and by the development of a more pointed umbo. 



/. subcardissoides, Schliit., I. digitatus, Sow., I. pinmfor mis, Will., 

 and /. corrugattis, Woods,- are allied one to the other, and appear to 

 have arisen from the less convex forms of /. lamarchi by the develop- 

 ment of radial folds such as occur occasionally in /. lamarchi and 

 allied species. 



B. /. crippsi, Mant. (Upper Greensand to zone of H. sulglolosus), 

 agrees in many respects with the form of /. angliciis, which has fewer 

 and less regular ribs, but in this species the hinge has become 

 somewhat shorter, the postero-dorsal part of the shell less compressed, 

 the anterior area smaller, and the ribs fewer and more irregular with, 

 a less strongly marked posterior curvature. 



In /. crippsi, var. reachensis, Eth. (Chalk Marl to zone of H. suh- 

 glohosus), the shell has become relatively higher and the posterior 

 curvature of the ribs greater : this variety resembles closely the 

 small forms of /. lahiatus (Schloth.) (zones of Rh. cuvieri and T. lata) ; 

 but in later stages the shell in that species grows extensively in 

 a direction oblique to the hinge, and thus acquires a mytiliform shape. 

 In the zone of Terebratidina lata the specimens of /. lahiatus are 

 usually longer and less high than in the zone of Rh. cuvieri, and these 

 pass into /. labiatus, var. lattis, Sow. {H. planus zone). 



/. inconstans, Woods' (zone of H. planus to zone of B. mucronata), 

 undoubtedly belongs to the lahiatus stock, and the nearly flat forms 

 approach closely /. labiatus, var. latus, from which they have probably 

 been derived. 



I, halticus, Bohm (zone of Marsupites to zone of B. mucronata), 

 possesses many of the characters of /. iticonstans, from which it has 

 developed by an increase in the length of the hinge. 



/. lingua, Goldf. (zones of Marsupites and A. quadratus), is similar 

 to the nearly flat forms of /. inconstans, but the shell has become 

 higher. /. lobatus, Goldf. (zones of Marsupites and A. quadratus), is 

 near to /. lingua, but has developed an angular ridge between the 

 umbo and the postero-ventral extremity. /. cardisscides, Goldf. (zone 

 of A. quadratus), and /. tuber culattts, Woods* (zone of A. quadratus), 

 are closely related to /. lobatus. 



I. nndulato-plicatus, Rom. (Upper Chalk), resembles the nearly flat 

 forms of /. inco7istans, from which it appears to have been derived by 

 the development of diverging radial folds. 



1 Q.J.G.S., vol. liii, pi. xxvii, figs. 14-17, 1897. 



2 Type, British Museum, L 22528. 



^ G. A. Mantell, Foss. S. Dotvns, pi. xxvii, fig. 9, and pi. xxviii, fig. 3, 1822 ; 

 and F. Dixon, Geology of Sussex, pi. xxviii, fig. 29, 18-50. 



^ Type in Dr. A. W. Rowe's collection, from Sewerby (Yorkshire). 



