NORTH-CAROLINA GEOLOGICAL SURVEY. 



DONAX. (Fig. 226.) 

 FIG. 226. Shell triangular, rather abruptly truncate be- 



hind, and traversed by a ridge from the umbo to 

 the base ; surface marked by obscure radiating 

 lines; base crenulated. This small shell differs 

 from the variabilis in its proportion; it is more triangular, and 

 is not produced so much in front. 



Donax Variabilis probably occurs in the marl of Torth- 

 Carolina, but has hitherto been overlooked. 



FAMILY MACTRIDAE. GENUS MACTRA. 



" The shell is equivalve, and nearly equilateral ; the ante- 

 rior hinge tooth is in the form of an inverted A ; lateral teeth 

 doubled in the right valve." 



MAOTRA CONGESTA. 



Shell rather small, but thick at the umbo ; triangular, rath- 

 er inflated ; inequilateral ; rounded anteriorly, and posterior- 

 ly it is produced. Yery common in the marl of Wayne and 

 Edge combe. 



MACTRA LATERALIS. SAY. (Fig. 227.) 



FIG. 227. Shell small, rather thin, smooth, sub-tri- 



angular ; lines of growth fine ; posterior side 

 elongated, or margins sub-equal, rounded 

 before ; umbo rather prominent. A very 

 common fossil of the miocene. 



4* 



MACTRA SIMILIS. SAY. 



.Shell thin, of a medium size, margins sub-equal, concentric, 

 striae very fine, at intervals deep, beaks nearly central. The 

 living ones of the coast have a longitudinal rounded ridge 

 running from the beaks to the base and obscure radiating 

 lines, though only visible in a favorable position. 



' GNATUODON GRAYII. (Fig. 226a.) 



Shell rather thick, sub-triangular, inflated, inequilateral, 



