MOLLUSCA. 461 



i860. Trigonia thoracica Gabb, Jour. Acad. Nat. Sci. Phil, 

 2nd sen, vol. 4, p. 304, pi. 47, fig. 10. 



i860. Trigonia thoracica Con., Jour. Acad. Nat. Sci. Phil., 

 2nd sen, vol. 4, p. 304, pi. 47, fig. 10. 



1861. Trigonia thoracica Gabb, Synop. Moll. Cret. Form., p. 



233 (177)- 

 1864. Trigonia thoracia Meek, Check List Inv. Foss. N. A., 



Cret. and Jun, p. 9. 

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

 1870. Trigonia limbata Credner, Zeitsch. Deutsch. Geol. Ges., 



Band 22, p. 234. 

 1876. Trigonia thoracica Gabb, Proc. Acad. Nat. Sci. Phil. 



(1876), p. 312^. 

 1886. Trigonia MortonfWhiti., Pal. N. J., vol. i (Monog. 



U. S. G. S., vol. 9), p. 112, pi. 14, figs. 5-6. 

 1898. Trigonia thoracica Johns., Proc. Acad. Nat. Sci. Phil. 



(1898), p. 464. 

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



(1905), p. II. 



Description. — Shell large, the dimensions of an average sized 

 individual being: length, 48 mm.; height, 42 ram.; convexity, 

 II mm. Ovate subtrigonal in outline, the valves moderately 

 convex in front, becoming compressed posteriorly; the beaks 

 nearly anterior, slightly recurved. Anterior margin broadly 

 rounded, passing intO' the ventral margin; ventral margin 

 broadly rounded, often becoming a little straightened as it ap- 

 proaches the posterior extremity of the shell; posterior margin 

 obliquely subtruncate above; dorsal margin gently concave from 

 t)he beak tO' the posterior hinge extremity. Surface of the 

 valve divided intO' two- portions by an angular, curved furrow, 

 passing backward from just behind the beak sub-parallel with 

 the dorsal margin, to a point in the posterior margin of the 

 shell a short distance below the posterior hinge extremity; the 

 lower portion of the valve constitutes much the greater part and 

 is marked by about fifteen ribs, about ten of which are very 

 strong, subangular, more or less nodose, with broad concave 



