402 CRETACEOUS PALEONTOLOGY. 



side of the beak, 20 or more in number. Surface of the valves 

 as indicated by impressions of the exterior marked only by more 

 or less indistinct lines of growth. 



Remarks. This seems to be a very distinct species of Trigon- 

 arca which is characterized by the slight elevation of the beaks 

 above the hinge-line, and in the shell itself must have had very 

 low cardinal areas. All the New Jersey specimens observed are in 

 the form; of internal casts. 



Formation and locality. Cliffwood clay, Cliffwood Point 

 (185). 



Geographic distribution. New Jersey, North Carolina. 



Trigonarca triquetra Conrad. 



Plate XXX., Fig. 16. 

 1875. Trigonarca iriquetra Con., Kerr's Rep. Geol. Surv. N. 



Car., App. A, p. 2, pi. i, fig. 7. 

 1872. Con., Proc. Acad. Nat. Sci. Phil., 1872, 



pi. 2, fig. 5. 



Description. Shell subtriangular in outline, the dimensions of 

 a right valve being : length, 41 mm. ; height, 32 mm. ; convexity 

 of one valve, 8.5 mm. Hinge-line strongly arcuate; anterior 

 margin broadly rounded, passing regularly into the gently con- 

 vex basal margin ; postero-basal margin rather sharply rounded ; 

 posterior margin obliquely truncate. The valves moderately 

 convex, the beaks situated near the middle of the hinge-line and 

 but slightly elevated above it; umbonal ridge rounded, the post- 

 umbonal slope gentle below, becoming more abrupt towards the 

 beak. Indentations of both anterior and posterior muscular 

 ridges present in the casts, the anterior indentation very slight, 

 posterior one much stronger, its lower end below the mid-height 

 of the shell. Pallial impression distinct, above which the surface 

 of the cast is marked by distinct radiating grooves and costae. 

 Hinge strongly arcuate, teeth short, diverging from either side 

 of the beak, thirty or more in number. Surface of the valves, as 

 indicated by impressions of the exterior, marked by fine, crowded, 

 concentric lines of growth, and by indistinct, rather broad and 

 flat radiating costse upon the central portion of the shell. 



