110 ON THE SECOxNDARY AND TERTIARY FORMATIONS 



MACTRA. 



Mactra crassidcns. Plate V, fig. 11. Triangular, thick, convex- 

 dei)ressed ; umboidal slope submarginal, angulated : beaks central ; 

 cardinal plate thick, lateral teeth robust. This species is from JMr. 

 Smith's collection. It belongs to the genus Mulinia of Gray. 



M. subparilis. Plate V, fig. 12. Triangular, elongated, moderately 

 tliick, convex-depressed : posterior side cuneiform ; apex hardly 

 oblique, subcentral ; fosset wide ; lateral teeth transversely striated. 

 Belongs to the genus Spisula, Gray. 



LUCINA. 



Lucina trisulcata. Obovate, convex; with concentric lines, and 

 two or three distant concentric furrows ; lunule profound. Difi^ers 

 from L. alveola of the lower tertiary, in being less ventricose, and in 

 the much more profoundly impressed lunule ; the cardinal teeth are 

 also very different. 



L. radians. Orbicular, convex, with minutely waved or rugose 

 concentric striae, and radiating striae obsolete on the middle of the 

 valves, and most distinct on the anterior side ; posterior submargin 

 destitute of radiating lines ; beaks central, prominent ; margin crenu- 

 lated. This species is recent at Mobile Point, Alabama, and occurs 

 near Newbern, North Carolina. 



MYTILUS. 



Mytilus incrassatus. Thick, much inflated ; anterior margin slightly 

 incurved near the middle ; basal margin not obtusely rounded ; hinge 

 thick, with slightly prominent, robust teeth. Length about three 

 inches. 



CARDIUM. 



Oardium suhlineatum. Plate V, fig. 13. Obliquely obovate, tliin, 

 shghtly ventricose, with obsolete radiating lines, «iost distinct near 

 the ends ; submargins of anterior and posterior sides destitute of 

 radiating lines ; within striated : margin crenulated. 



GNATHODON. 



Gnathodon minor. Plate V, fig. 14. Trigonal, convex-depressed, 

 posterior margin truncated and nearly direct. This makes the fourth 

 species of tliis interesting genus, two recent and two fossil. 



AMPHIDESMA. 



Amphidcsma constricta. Plate V, fig. 15. Oblong oval, ventricose ; 

 basal margin opposite the apex slightly contracted ; end margins 

 rounded ; beaks nearest the posterior extremity ; fosset profound ; 

 cardinal teeth prominent, lateral teeth none. 



