EUMETRIA 443 



very short. The dimensions of two individuals are : length of pedicle 

 valve 19 mm. and 21 mm., length of brachial valve 16 mm. and 18 mm., 

 greatest width 18 mm. and 19 mm., thickness 11 mm. and 14 mm., length 

 of hinge-line 5 mm. and 5 mm. 



Pedicle valve more or less strongly convex, the greatest convexity pos- 

 terior to the middle, the umbonal region prominent and projecting not- 

 ably beyond the hinge-line, the surface curving from the umbonal region 

 to the anterior with a regularly decreasing convexity, laterally the curva- 

 ture becomes more abrupt towards the cardinal extremities; towards the 

 cardinal margin the surface is inflected and not infrequently slightly 

 auriculate at the cardinal extremities ; the beak is pointed, strongly in- 

 curved, and is pierced by a large, subcircular foramen which encroaches 

 notably upon the umbonal region, only the margin of it occupying the 

 apical portion of the delthyrium; the cardinal area is very small, arched, 

 the lateral margins usually sharply defined and sloping in nearly straight 

 lines from the sides of the beak to the cardinal extremities, the delthy- 

 rium closed by a pseudodeltidium ; the mesial sinus is obsolete, but in some 

 individuals there is a slight, indefinite flattening in the anterior median 

 portion of the valve. 



Brachial valve a little less convex than the pedicle, the greatest convex- 

 ity posterior to the middle ; the beak pointed and very slightly projecting 

 beyond the hinge-line; the surface curving from the beak to the anterior 

 margin with gradually lessening curvature, laterally the curvature be- 

 comes more abrupt towards the cardinal extremities where the surface is 

 compressed to form small auriculations which are larger than those of the 

 pedicle valve ; along the median portion of the valve the surface is slightly 

 and indefinitely flattened towards the front of the shell, but no mesial 

 fold is developed, the flattening, however, is usually more marked than 

 that of the pedicle valve. 



The surface of both valves is marked by fine, simple, rounded, nearly 

 uniform, radiating plications, separated by furrows narrower than the 

 plications themselves ; each valve bears from 42 to 55 of these plications, 

 the more usual number being between 46 and 50. The shell structure is 

 minutely punctate. 



Remarks. This species exhibits great variation in size, in the propor- 

 tional dimensions of length and width, and in the convexity of the valves. 

 Some examples attain a larger size than either of those whose dimensions 

 have been given above ; a smaller size is the more usual, however, although 

 the specimens exhibit the characteristics of adult shells. Occasionally 

 the valves become strongly ventricose, especially in old individuals. 



In his bibliographic list Schuchert 1 has included this form as well as 

 E. vera Hall and E. vera var. costata Hall, as synonyms under E. marcyi 



1 Bull. U. S. Geol. Survey, No. 87, p. 222. (1897.) 



