MOLLUSCA. 409 



the anterior third of the entire length of the valves. Cardinal 

 area moderate in size and extending about two-thirds of the 

 length. Teeth unknown. Surface as indicated on internal casts 

 marked by radiating ribs, pretty fine and numerous on the anterior 

 end and in the mesial sulcus, becoming much coarser posteriorly, 

 and showing a slight tendency to alternation in size between the 

 sulcus and posterior umbonal angle, and on the cardinal slope a 

 tendency to bifurcation in some cases." (Whitfield.) 



The dimensions of a perfect internal cast are : length, 38 mm. ; 

 height, 18.5 mm.; thickness, 13 mm. 



Remarks. No examples of this species have been met with in 

 the recent collections of the Survey. The most perfect individual 

 illustrated by Whitfield has the general outline of Mprton's type 

 from Alabama, but 'is not so conspicuously marked by radiating 

 costae, and has not so broad a byssal gap in the ventral margin. 



Formation and locality. Navesink marl, Holmdel (Coll. N. J. 

 Geol. Survey). 



Geographic distribution. New Jersey. 



Area obesa (Whitfield). 

 Plate XXXIV., Fig. 9. 



1886. Cibota obesa Whitf., Pal. N. J., vol. i (Monog. U. S. G. 



S., vol. 9), p. 93, pi. n, figs. 30-31. 

 1905. Cibota obesa Johns., Proc. Acad. Nat. Sci. PJiil., (1905), 



p. 9. 



Description. "Shell small, with full and very ventricose valves, 

 large tumid beaks situated opposite the anterior third of the 

 length, slightly enrolled, and distant from each other as shown on 

 the internal cast. Form of the outline trapezoidal, the length of 

 the cast nearly twice the height, exclusive of the projection of 

 the beaks; anterior end vertically rounded; posterior obliquely 

 truncate; extremity obtusely pointed; basal line full, but con- 

 stricted just anterior to the middle by the very marked but short 

 and broad byssal opening; area two-thirds the length of the valve 

 and moderately wide. On the casts the muscular imprints are 

 very distinctly marked and of fair size, no muscular ridge; the 



