EXPLANATION OF PLATE 13 



All figures natural size. 



Fig. la. Spisula solidissima Dilliwyn. Atlantic Coast, Recent. 

 The type of the subgenus Hemtmactra. 



Fig. lb. Spisula solidissima Dilliwyn. Atlantic Coast, Recent. 



Fig. 2. Mactra cxoleta Gray . Pleistocene. 



Fig. 3. Mactra trampasensis, n. sp. Miocene. 

 Umbonal view. 



Fig. 4. Spisula voyi (Gabb). Pliocene. 

 Probably Gabb's type. 



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