Vol. 68.] THE EVOLUTION OF INOCERAMUS. 11 



from the lamarcki type, this valve becomes nearly flat (fig. 47, 

 p. 10), the anterior area is lost, and the concentric folds increase in 

 strength ; the height of the shell decreases, and is often exceeded 

 by the length. 



From the large size and great thickness of the shell, and from 

 the great difference in the form and size of the two valves, it seems 

 evident that the characters which distinguish /. involutus from 

 /. lamarcki were developed by the adoption of a more sedentary 

 mode of life in which the animal rested on its left valve. The 

 opercular character of the right valve, due to its decreased convexity 

 and to the marginal growth around the hinge, ^ is in accordance with 

 this view. Reasons for the short zonal range of /. involutus may 

 be seen in the great size and thickness of the shell and in its 

 specialized character. 



I. involutus has been placed by Stoliczka and some later writers 

 in a separate genus or subgenus — Volviceramus ; but, since that 

 species has been shown to have descended from /. lamarcki, the 

 separation which would be implied by the use of the name 

 Volviceramus can no longer be maintained. 



/. cordiformis Sow. (figs. 51-53, p. 10) is found in the zones of 

 Micraster cor-testudinarium and M. cor-a7iguinum, and in the 

 Uintacrinus Band of the zone of Marsupites testudinarius, also 

 possibly in an earlier zone. It has been derived from an inflated 

 form of /. lamarcki with nearly equal valves, with which it is con- 

 nected by an intermediate variety (figs. 49 & 50). It differs from 

 /. lamarcki in the equal size of the valves, the relatively longer 

 hinge, the presence of a radial sulcus extending from behind the 

 umbo to the postero-ventral extremity, and often also of another 

 sulcus from the front of the umbo to the opposite ventral margin ; 

 when this sulcus is present the large anterior area is limited by a 

 rounded ridge. In one variety of /. lamarcki (figs. 49 & 50) this an- 

 terior sulcus is found, but the shell is shorter than in /. cordiformis. 



I. costellatus Woods (figs. 54 & 55, p. 10) is found only in the 

 Chalk Rock (zone of Heteroceras reussianum), and has probably 

 been derived from /. lamarcki var. aj^icalis by the axis of growth 

 becoming more oblique to the hinge, accompanied by the develop- 

 ment of a narrower and more prominent left umbo. 



I. suhcardissoides Schliit.,^ 1. digitatus Sow.,^ /. pinniformis 

 Will.,^ and /. con^ugatus Woods,^ from the Upper Chalk, are allied 

 to one another and possess strong radial folds. No perfect specimen 

 of these species showing the umbo and hinge has yet been found ; 

 but, from the character of the posterior ear and the curvature of the 

 concentric folds, it seems probable that they have been derived from 



^ H. Woods, op. cit. fig. 94. 



2 Palaeontographica, vol. xxiv (1877) p. 271 & pi. xxxvii. 



^ H. Woods, op. cit. text-fig. 95. 



* Id, ibid, text-fig. 96. s Id. ibid, text-fig. 97. 



