MO'LLUSCA. 515 



portions; in the middle of the shell every other costa on large 

 individuals has usually been intercalated between two others at 

 some distance below the beak ; the shell is also marked by more 

 or less irregular, concentric undulations. 



Remarks.- — ^This species is one of the most characteristic mem- 

 bers of the Merchantvile clay-marl fauna, where it sometimes 

 occurs in considerable numbers. In all other formations where 

 it occurs in New Jersey, it is exceedingly rare. The species seems 

 to be a perfectly typical species of Pholadomya in all respects. 



Pormation and locality.- — Cliffwood clay, Cliffwood Point 

 (105), near Matawan (107); Merchantville clay-marl, near 

 Matawan (loi), near Jamesburg (139, 141), Ivenola (163); 

 Woodbury clay, near Matawan (103). , 



Geographic distribution. — New Jersey, Alabama, Mississippi, 

 Arkansas. 



Pholadomya roemeri Whitfield. 



Plate LVI., Figs. 4-5. 



1886. Pholadomya Roemeri Whitf., Pal. N. J., vol. i (Monog. 

 U. S. G. S., vol. 9), p. 176, pi. 24, fig. 4. 



Description. — ^The approximate dimensions of a rather small 

 specimen are: length, 38 mm.; height, 22 mm.; thickness, 18 

 mm. Shell very oblique and inequilateral, elongate subovate in 

 outline, widest back of the middle. Beaks small, incurved and 

 nearly in contact, situated far forward. Hinge-line straight, 

 rather long; anterior margin rounding from the anterior car- 

 dinal extremity into the basal margin; basal margin gently con- 

 vex, curving upward posteriorly ; posterior margin rather sharply 

 rounded above the mid-height of the shell. Valves strongly con- 

 vex or ventricose, the surface curving rather abruptly from the 

 prominent umbones to the dorsal, anterior and ventral margins, 

 much more gently to the posterior margin. Surface of each valve 

 marked by about 13 narrow, angular, radiating costse, separated 

 by broad, concave interspaces; the most anterior costss curve 

 slightly forward in passing from the beak to the margin of the 



