MOLLUSCA. 463 



1868. Trigonia Eiifalensis Con., Cook's Geol. N. J., p. 725. 

 1886. Trigonia Eufaulensis Whitf., Pal. N. J., vol. i (Monog. 



U. S. G. S., vol. 9), p. 113, pi. 14, figs. 1-4. 

 1905. Trigonia eufalensis Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. n. 



Description. Shell small, the dimensions of an average speci- 

 men being: length, 21 mm.; height 15 mm.; convexity, 4 mm. 

 The largest specimen observed is under 30 mm. in length. Ovate 

 subtrigonal in outline, somewhat alate posteriorly, moderately 

 convex in front, compressed behind. Beaks almost anterior, 

 slightly recurved. Anterior and antero-basal margin broadly 

 rounded, postero-basal margin nearly straight, sloping upward 

 towards the posterior hinge extremity, posterior extremity 

 rounding sharply into the dorsal margin; dorsal margin nearly, 

 straight behind, becoming more strongly concave as it approaches 

 the beak. Surface of the valves divided into two portions by a 

 ridge passing with a concave curve from the posterior side of the 

 beak to the posterior margin of the shell just below the posterior 

 extremity of the hinge-line. The lower portion of the valve is 

 marked by 12 or 14 strong, angular, non-nodose ridges, nar- 

 rower than the interspaces, the more anterior ones of which curve 

 strongly forward in passing? from the bounding ridge to the shell 

 margin, the more posterior ones becoming straighter, in some 

 cases having a slightly sigmoidal curve. The upper portion of 

 the shell is inflected above the bounding ridge for about one- 

 lialf the distance to the hinge-margin, above which it is again 

 deflected into nearly a plane with the valve, the ribs of the lower 

 portion of the shell are continued across the upper portion, being 

 abruptly bent backwards in crossing the bounding ridge, the more 

 posterior ones being more strongly bent than those in front. 

 Besides the ribs, the shell is marked by inconspicuous lines of 

 growth. 



Remarks. This species usually occurs in the New Jersey 

 formations in the form of more or less indefinite internal casts, 

 but occasionally in certain hard nodules good impressions of the 

 exterior are preserved, from which casts may be taken to show 

 the external characters. It has been from such casts that the 



