THE EV0LT7TI0N OP INOCEJRAMUS. 



[Feb. 19 1 2. 



I. Species connected with Inocebamus salomoni. 



We may first deal with the species which are connected 

 with I. salomoni, namely : — /. concentricus, I. concent licns var, 

 suhsulcatus, I. sidcatus, I. tenuis, and /. etheridgei, of which the 

 distribution and relationship are shown in the accompanying 

 diagram (A). 



Diagram A. — Distribution and relationship of the species connected 

 luith Inoceramus salomoni. 



ZONES 



Holaster 

 sulgloljosus 



ethei 



idgei 







Schlceuhachi 



(Hystrichorcras') 

 variant 





tei 



luis ■rf 



Xi 



Pecten a^iper 







concentricus > 



sulcatus 



1 





UpperGreensand, 





^""""^ 





1 





Gaiilt,and 









Red Chalk 







salomoni 





Duarilliiri'ras 



DhntiuullatNVi 





Folkestone Beds 

 (Aptian) 





[The thick lines express the range of the species ; tlie thin bnes connecting 

 them indicate what is believed to be the phylogeny of the species.] 



/. salomoni d'Orb. (figs. 1-4, p. 3) is found in the Folkestone Beds 

 (Lower Greensand) and in the Mammillatum Bed at the base of the 

 Gault. The left valve is characterized by its subquadrate outline, 

 the obliquely triangular hinge-area, the large anterior area 

 bounded by a rounded ridge, the shallow radial sulcus which 

 extends from below the umbo to the ventral margin and gives a 

 sinuosity to the concentric ridges. In the early stage of growth 

 the sulcus is not present, and the period at which it appears varies 

 in different individuals. 



.7. concentricus Park. (figs. 5-9, p. 3) ^ ranges throughout the 

 Gault and the Upper Greensand (with the exception of the zone 



1 For otiier figures, see H. Woods, ' Monogr. Cret. Lamellibr.' (Palisont. Soc), 

 vol. ii (1911) pi. xlv, fig. 11, pi. xlvi, figs. 1-10, & pi. xlvii, figs. 1-2. 



