90 Trans. Acad. Sci. 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 



