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CRETACEOUS LAMELLIBRANCHIA. 



several species which are placed by all writers in the genus Inoc&ramus. In this 

 respect Volviceramus is exactly comparable with Actinoceramus (p. 2GS). 



A fairly complete passage can be traced from I. Lamarclci to I. involutus. In 

 some forms of I. Lamarclci the valves become more unequal than usual, the left 



/ 



Fu;. 93. — Inoceramus involutus, Sow. Upper Chalk, locality unknown. British Museum, No. L21003. 



Anterior view of left valve, x |. 



valve being relatively larger and with less distinct folds, and the right relatively 

 less convex and its anterior area slightly concave (Fig. 85). These are connected 

 with some varieties of /. involutus by intermediate forms (Figs. 86, 87) in which 

 the left valve possesses nearly all the characters of I. involutus but is less curved 

 and possesses a concave anterior area, whilst on the right valve the folds are 

 almost as prominent as in I. involutus, but the flattened or concave anterior area 



