Vol. 68.] THE EVOLUTION OP INOCERAMUS, 13 



the less convex forms of /. lamarcki with concentric folds. That 

 radial folds or corrugations, which give greater strength to the 

 shell, can be developed in that stock is shown by the occasional 

 presence of a fold in /. lamarcki, by the folds in /. cordiformis^ and 

 still more by the fold in /. lezennensis Decocq ^ (fig. 48, p. 10). 



/. crippsi Mant. (figs. 58 & 59, p. 12) "^ ranges from the Up])er 

 Green sand to the zone of Holaster suhglohosus. It appears to have 

 been derived from the forms of /. anglicus which possess fewer and 

 less regular ribs (figs. 56 & 57). It agrees in many respects with 

 /. anglicus, but the shell is relatively shorter, the part of the valve 

 between the umbo and the ventral margin is less convex (so that 

 the postero-dorsal part of the shell is relatively less compressed), and 

 the anterior area is smaller ; the ribs are fewer, more irregular, 

 and their posterior curvature towards the hinge-line is less strong. 

 In I. crippsi var. reachensis Eth. (fig. 60),^ ranging from the Chalk 

 Marl {varians zone) to the zone of Holaster suhglohosus, the shell, 

 when fully grown, is relatively higher, and the posterior curvature 

 of the ribs is greater than in /. crippsi. 



I. lahiatus (Schloth.) (figs. 61-63, p. 12) ^ is found commonly in 

 the zone of BTiynchonella cuvieri, and also occurs in the zone of 

 Terehratulina lata. The curvature of the ribs, the similarity of the 

 anterior area, and the fact that the anterior margin of the shell is 

 at first perpendicular to the hinge, ^ make it most probable that 

 this species has descended from /. crippsi var. reachensis. The main 

 differences which appear in /. lahiatus are due to the extensive 

 growth of the shell in a direction oblique to the hinge-line, so that 

 in fully-grown specimens (fig. 62) the form appears very different 

 from that of /. crippsi. On account of this mode of growth, the 

 resemblance between small specimens of /. lahiatus (fig. 61) and 

 the adult of /. crippsi var. reachensis is greater than in older 

 specimens. Unfortunately, but few perfect specimens of I. crippsi 

 and its variety reachensis have yet been found, so that their varia- 

 tion cannot be studied satisfactorily ; and no forms intermediate 

 between this species and /. lahiatus have been seen. 



/. lahiatus var. latus Sow. (fig. 64, p. 12) ^ occurs in the zone of 

 Holaster plamis. The specimens of /. lahiatus found in the zone 

 of Terehratulina lata are often relatively longer and less high than 

 the examples which occur in the zone of Rhynchonella cuvieri, and 

 such forms pass gradually into /. lahiatus var. latus, in which the 

 length of the hinge-line has increased in proportion to the height 

 of the shell ; the axis of growth has become less oblique, the 

 anterior margin more convex, and the ventral margin less sharply 

 curved. 



1 C. Barrois, Ann. See. Geol. Nord, vol. vi (1879) p. 455 & pi, v, figs. 1-2. 



2 H. Woods, op. cit. pi. xhii, figs. 2-3 & text-figs. 33-35. 

 •■' Id. ibid. pi. xlviii, figs. 4-5 & pi. xlix, fig. 1. 



4 Id. ibid. pi. 1 & text-fig. 37. 



5 Id. ibid. pi. 1, fig. 4. 



6 Id. ibid, text-figs. 38, 40, & 41 {non 39). 



