Vol. 6S.'] THE EVOLTJTION OF INOCERAMVS. 7 



I. pictus Sow. (figs. 30 & 31, p. 8) ^ ranges from the Chalk Marl 

 (yarians zone) to the zone of Holaster suhglohosus ; it resembles the 

 forms of /. anglicus which have more numerous and more regular 

 ribs (figs. 28 & 29). In /. pictus the ribs have become smaller, more 

 numerous, and more regular ; the anterior area is better developed, 

 and is concave or nearly flat ; and the shell is relatively shorter and 

 higher than in /. anglicus. 



I. lamarcki Park. (figs. 32-41, p. 8) begins in the zone of 

 lihynclionella cuvieri and extends to that of Micraster cor-anguinum. 

 Some early forms of this species (/. lamarcki var. apicalis Woods, 

 figs. 32 & 33),^ in which the valves are only slightly unequal, 

 the concentric folds not much developed, and the posterior ear 

 not sharply limited, closely resemble /. pictus, and seem almost 

 certainly to have descended from that species.^ /. lamarcki shows 

 a great amount of variation,* so that many of the forms have 

 diverged considerably from 1. jpictus owing to change in convexity, 

 in the number, strength, and curvature of the concentric folds, in 

 the size and distinctness of the posterior ear, and in the relative 

 height of the shell, etc. 



In /. lamarcki var. cuvieri Sow. (figs. 40 & 41),' which ranges 

 from the zone of Terehratulina lata to that of Micraster cor- 

 anguinum, the shell is high, with the valves only slightly convex 

 and not very unequal, and with the concentric folds often absent 

 or indistinct though sometimes becoming more or less strongly 

 developed. 



In forms like the type of /. lamarcki Park. (figs. 34-36J the 

 valves are inflated and usually very unequal, the umbones prominent, 

 the posterior ear sharply defined, and the concentric folds well- 

 developed. Other similar forms have the posterior ear indistinctly 

 limited, and the folds may be only poorly developed. The left valve 

 is sometimes very much more convex than the right (fig. 38), and 

 occasionally the two valves are of nearly equal convexity (fig. 37). 



/. lamarcki var. websteri Mant. (fig. 39) occurs mainly in the 

 zone of Micraster cor-testudinarium ; it is an inflated form with the 

 posterior ear not sharply limited, and is distinguished by the very 

 thin shell and the sharp ridge-like character of the concentric folds. 



I. involutus Sow. (figs. 44-47, pp. 9 & 10), found in the zone of 

 Micraster cor-testudinarium and in the lower part of the zone of 

 M. cor-anguinum, is shown by intermediate varieties (figs. 42 & 43, 

 p. 9) to have descended from a convex form of /. lamarcki, by the 

 increase in size and convexity of the left valve, its acquirement of a 

 spirally curved umbo, the loss of concentric folds, and the loss of the 



^ H. Woods, 02). cit. pi. xlix, figs. 5 & 6. 

 2 Id. ibid. (1912) pi. liii, figs. 4-6. 



^ The resemblance is greater in the case of small forms of /. pictus than in 

 the large example figured here. 

 * Id. ibid. pi. lii, figs. 4-6, pi. liii, figs. 1-7, & text-figs. 63-85. 

 5 Id. ibid, text-figs. 73-84. 



