Reports and Proceedings — Geological Societi/ of London. 43 



dykes, which may possibly be of the same age as the acid lavas 

 below the conglomerate of Eastern Jersey, now regarded as basal 

 Cambrian ; but in Alderney two diabase dykes cut the gres 

 i'eldspatliique. There is, however, reason to think that these dykes are 

 of more than one age. The mica-traps are probably late Palseozoic. 



2. "The Evolution of Inoceramus in the Cretaceous Period." 

 By Henry Woods, M.A., F.G.S., Lecturer in Palseozoology in tlie 

 University of Cambridge. 



The species of Inoceramus found in the Gault, the Upper Greensand, 

 and the Chalk are considered to have descended from two stocks wliich 

 occur in the Lower Greensand, one being /. salomoni, d'Orb., the othei' 

 of the type of /. neocomiensis, d'Orb. 



I. /. concentricus, Park. (Lower and Upper Gault), is of the same 

 type as /. salomoni (Folkestone Beds and Mammillatuni Bed), from 

 which it has been derived by the reduction in the length of the 

 hinge-line and in the height of the hinge-area, accompanied by 

 a greater obliquity of the axis of growth. 



I. sulcatus, Park. (Upper Gault), closely resembles /. concentricus, 

 except that it possesses strong radial ribs. Between these two species 

 everj^ gradation is seen in the form known as /. concentricus, var. 

 suisulcattis, Wiltsh. (Upper Gault), which in its early stage is identical 

 with /. concentricus, but at a later stage develops radial folds ; the 

 eojicentricus stage may be of long or short duration, and consequently 

 in some cases the folds occur near the margin of the valves only, but 

 in others may reach to near the umbo, while in /. stdcatus they reach 

 to the apex of the nmbo. 



/. tenuis, Mant. (Red Chalk and Chalk Marl), is allied to 

 /. concentricus, from which it has been derived by an increase in 

 the length of the hinge and a decrease in the prominence of the left 

 umbo. I. etheridgei, Woods (zone of P. asper to zone oiH. suhglohosus), 

 is similar to /. tenuis, but its left umbo is smaller, the valves less 

 unequal, and the postero-dorsal region is less compressed. 



II. /. anglicus, Woods (Gault and Upper Greensand), resembles 

 I. neocomiensis, but the posterior part of the shell has become more 

 compressed, and the ventral curvature of the ribs has increased. 

 From /. anglicus two species appear to have arisen, namely, /. pictus 

 and /. crippsi. 



A. I. pictus, Sow. (Chalk Marl to H. subgloiosus zone), approaches 

 the form of /. anglicus, which has more numerous and more regular 

 ribs ; and in it the ribs have become still more numerous and more 

 regular, and the anterior area has become more extensively developed. 



I. lamarcki, Park, (zone of lih. cuvieri to zone of M. cor-anguinum), 

 shows a great amount of variation. An early form has nearly equal 

 valves and only slightly developed concentzic folds, and approaches 

 closely /. pictus, from which it is believed to have been derived. 



I.involutus, Sow. (zones of M. cor-testudinarium and M. cor-anguinum), 

 is linked by intermediate forms with the variety of /. lamarchi, which 

 has a very convex left valve and prominent umbo. In the inter- 

 mediate forms the left valve loses its folds and develops a large, 

 spirally-coiled umbo ; the right valve becomes less convex and gradually 

 loses its anterior area, but retains the strong concentric folds. 



