298 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 diifers 

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

 is not produced so much in front. 



Donax Yariabilis probably occurs in the marl of North- 

 Carolina, but has hitherto been overlooked. 



FAMILY MACmiDAE. GENUS MACTRA. 



&quot; 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.&quot; 



MACTRA CONGESTA. 



Shell rather small, but thick at the urnbo; triangular, rath 

 er inflated ; inequilateral ; rounded anteriorly, and posterior 

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

 Edgecombe. 



MACTRA LATERALIS. SAY. (Fig. 227.) 



F IG . 227. Shell small, rather thin, smooth, sub-tri 



angular; lines of growth fine ; posterior side 

 elongated, or margins sub-equal, rounded 

 before ; urnbo rather prominent. A very 



common fossil of the miocene. 

 5 



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. 



GNATIIODON GRAYII. (Fig. 226a.) 



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



