THE LAMELLTBRANCHIA 63 



Atlantic. It is known fossil from the Lower Cretaceous 

 onwards, and was less restricted in its geographical range 

 until after the Eocene, as is shown by its presence in the 

 Barton Clay and (much more abundantly) in the Eocene 

 of the Paris Basin. 



6. Meretrix (Sinodia) incrassata (Fig. 18) is an 

 abundant fossil in the " Venus -bed " of the Lower 

 Oligocene of the Isle of Wight and Hampshire that 

 bed taking its name from the genus in which this species 

 was at one time included. 



FIG. 18. MERETRIX (SINODIA) INCRASSATA (J. SOWERBY), 

 OLIGOCENE. 



Interior of right valve. (Natural size.) (Original.) 



The shell is rounded and sub-triangular, prosogyral, 

 with fairly well marked lunule, without escutcheon, but 

 with a fairly long opisthodetic ligament. The surface 

 is ornamented with concentric striaB. The hinge-plate 

 is large and thick, and bears three cardinal teeth 

 in each valve, with two unequal anterior-lateral teeth 

 (Ai., Am.) in the right valve, and one (An.) in the left, 

 the hinge being of Cyrenoid type. The absence of 

 posterior-laterals is correlated with the length of the 



