MOLLUSCA. 607 



Genus MerETrix Lamark. 

 Meretrix tippana Conrad. 

 Plate LXVIIL, Figs. 1-3. 



1858. Meretrix Tippana Con., Jour. Acad. Nat. Sci. Phil., 2d 



ser., vol. 3, p. 326, pi. 34, fig. 18. 

 1861. Meretrix Tippana Gabb, Synop. Moll. Cret. Porm., p. 



198 (142). 

 1864. Dione tippana Meek, Check List Inv. Foss. N. A., Cret. 



and Jur., p. 13. 

 1868. Aphrodina Tippana Con., Am. Jour. Conch., vol. 4, p. 



246, pi. 18, fig. 5. 

 1868. Aphrodina Tippana Con., Cook's Geol. N. J., p. 727. 

 1886. Aphrodina Tippana Whitf., Pal. N. J., vol. i (Monog. 



U. S. G. S., vol. 9), p. 154, pi. 22, figs. 6-7. 

 1905. Aphrodina tippana Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. 16. 



Description. — The dimensions of a specimen from Ripley Mis- 

 sissippi, preserving the shell are : height, 28 mm. ; length, 35 

 mm. ; convexity, 8 mm. Shell subovate in outline. Antero- 

 cardinal margin concave in front of the beak, from in front of 

 this concave portion entirely around the shell to the posterior 

 side of the beak, the margin is convex. Beaks nearly central or 

 in front of the center of the shell, directed forward, scarcely in- 

 curved. Valves regularly convex, the surface curving more 

 abruptly to the cardinal margin, with a rather broad, scarcely 

 impressed lunule in front of the beak. Hinge of the left valve 

 with three cardinal teeth diverging from beneath the beak, the 

 anterior one curving forward and becoming thickened below, 

 the posterior one much more oblique than the others and more 

 elongate. Irr front of the cardinal teeth is a single weak lateral 

 beneath the lunule, parallel with the shell margin. In the right 

 valve are two divergent, cardinal teeth, the posterior one be- 

 coming thickened below with a longitudinal sinus, beneath the 

 lunule is a pit for the reception of the anterior lateral tooth of 

 the opposite valve. Surface of the shell marked by fine, more 

 or less regular, concentric lines of growth. 



