Geological Society. 275 



stocks which occur in tho Lower Grceusand, one being /. sctlomoni 

 d'Orb., tho other of the type of /. ncocomiensis d'Orb. 



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

 type us /. salomoiii (Folliestone Beds and Mammillatum Eed), from 

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

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

 greater obliquity of tlie axis of growth. 



/. sulcatus Park. (Upper Gault) closely resembles 7. concentricus, 

 except that it jiosscsses strong radial ribs. Between these two 

 species every gradation is seen in the form known as /. concen- 

 tricus var. sidisvlcatiis Wiltsh. (L'pper Gault), which in its eai'ly 

 stage is identical with /. concentricus, but at a later stage develops 

 radial folds ; the concentricus 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 tho umbo, while 

 in /. sulcatus they reach to the apex of the umbo. 



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

 centricus, 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. etheridfjci Woods (zone of P. asper to zone of H. sub- 

 glohosus) is similar to /. tenuis, but its left umbo is smaller, the 

 valves less unequal, and the postero-dorsal region is less com- 

 pressed. 



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

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

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

 From /. (tnglicus two species appear to have arisen, namely, I.pictus 

 and /. crijij^si. 



A. /. pictus Sow. (Chalk Marl to II. suhglohosvs 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. lamarcJci Park, (zone oi lih. cuvicri to zone of M. cor-anguinum) 

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

 valves and only slightly-developed concentric folds, and approaches 

 closely I. incius, 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 

 /. lamarcki which has a very convex left valve and prominent 

 limbo. In the intermediate 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 retain* the strong 

 concentric folds. 



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

 Band) has been derived from a variety of /. lamarcki with inflated 

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

 sulci and a relatively longer hinge. 



