4 THE EVOLUTION OF INOCEBAMUS. [Feb. I912. 



of Pecten asper), and is clearly of the same type as /. sdlomoni' 

 In comparing it with /. salomoni we find that the early stage is 

 similar, but of shorter duration ; and after that stage is passed the 

 left valve becomes more generally inflated, its axis of growth more 

 oblique, the umbo narrower and more pointed, the anterior area 

 smaller, the hinge-line shorter, and the hinge- area reduced in 

 height. The radial sulcus is not developed. /. concentricus appears 

 to have been derived from I. salomoni by the shortening of the hinge- 

 line and the reduction in height of the hinge-area. Some forms 

 of /. concentricus which possess a higher hinge-area (fig. 8, p. 3) 

 and a rather larger anterior area than usual connect /. concentricus 

 with /. sdlomoni. 



I. sulcatus Park. (figs. 19-21, p. 3) ^ occurs in the Upper Gault, 

 the Red Chalk, and the Upper Greensand (except in the zone of 

 Pecten asper)^ and agrees closely with I. concentricus^ notwith- 

 standing the presence of strong radial folds which, at first sight, 

 give it a very distinct appearance. Between these two species 

 numerous intermediate forms (known as /. concentricus var. suh- 

 sulcatus Wiltsh.) are found, in which every stage may be seen in 

 the development of the radial folds (figs. 10-18, p. 3). The early 

 part of the shell is identical with the adult shell of I. concentricus ; 

 in some cases this concentricus stage lasts for a considerable period 

 (figs. 12 & 13) ; while in others it is of short duration (figs. 15-18), 

 and in /. sulcatus (figs. 20 & 21) it is completely lost. The adult 

 in some examples differs from /. concentricus only in the possession 

 of a single radial fold (fig. 10) ; while in others two, three, or more 

 (figs. 11-18) develop, until we get a type like /. sulcatus, in which 

 the folds are sharp and numerous, and start from the apex of 

 the umbo (figs. 20 & 21). On account of its strong radial folds, 

 /. sulcatum has been placed by some authors in a separate genus 

 or subgenus — Actinoceramus, Meek ; but, since it has descended 

 directly from Inoceramus concentricus^ there is no reason for 

 assigning it to a separate genus or subgenus. 



/. tenuis Mant. (figs. 22 & 23, p. 5)^ ranges from the Red Chalk 

 ( = Gault) to the Chalk Marl (varians zone), and possesses most of 

 the features of /. concentricus, from which it has clearly been 

 derived ; the main differences are its longer hinge-line and the less 

 prominent umbo of the left valve. 



/. etlieridgei Woods (figs. 24-26, p. 5) ^ extends from the zone of 

 Pecten asper to the zone of Holaster suhglohosus, and appears to have 

 descended from /. tenuis. It possesses a similar concave anterior 

 area and long hinge. The chief differences are found in the left 

 valve, in which the umbo is less prominent, the postero-dorsal region 

 is less compressed, and the valves are more nearly equal. 



H. Woods, op. cit. pi. xlvii, figs. 15-20. 

 Id. ibid. pi. xlviii, fig. 1, & text-figs. 31-32. 

 Id. ibid. pi. xlix, figs. 2-4. 



