90 Trans. Acad. ScL of St. Louis. 



anteriorly. A shallow sinus extends from the beak obliquely 

 across the antero-ventral slope to near the center of the basal 

 margin. Surface marked by fine lines of growth which are 

 most conspicuous upon the antero-ventral slope. 



Remarks. The dimensions of this species as given by 

 Winchell are somewhat greater than those recorded above, 

 the length from the beak to the postero-ventral angle being 

 32 mm., and the hinge-line 20 mm. The length of the hinge- 

 line was therefore proportionately longer than in the specimen 

 here described. The species varies considerably in its pro- 

 portions, the two specimens illustrated being representative. 



Macrodon cochlearis Win. 



PI. III. f. 15. 



Shell subrhomboidal in outline, length, 23 mm. and height, 

 10 mm. Hinge-line straight, a little shorter than the total 

 length of the shell ; anterior margin meeting the hinge-line 

 at nearly a right angle, curving regularly downward and back- 

 ward into the ventral margin ; ventral margin nearly straight 

 or gently curving, except towards the extremities where it 

 curves upward; posterior margin abruptly rounded below, 

 truncate above, meeting the hinge-line in obtuse angle. The 

 valves rather ventricose; beaks flattened, incurved, elevated 

 above the hinge-line, situated at a point about one-fourth the 

 length of the shell from the anterior extremity ; the umbonal 

 ridge rather sharply rounded near the beak, becoming more 

 broadly rounded posteriorly, the dorsal slope from the ridge, 

 concave. Surface marked by concentric lines of growth. 



Macrodon modesta ( Win . ) . 



PI. III. f. 16. 



Area modesta. Bull. U. S. G. S. No. 153: 91. 



Shell small, ventricose, subovate in outline, length, 10 mm. 

 and breadth, 5 mm. Hinge-line straight, nearly equaling the 

 greatest length of the shell ; anterior abruptly rounded; ven- 

 tral margin broadly rounded; posterior margin broadly 

 rounded from the ventral margin to the hinge-line, which it 

 meets at an acute angle. Beak prominent, obtuse, situated 

 near the anterior extremity of the shell, elevated above the 



