EUMETRIA 445 



upon the umbonal region, only tne margin oi n oeing in contact with the 

 delthyrium; cardinal area small, arched, the lateral margins sharply 

 defined, sloping from the sides of the beak in nearly straight lines to the 

 cardinal extremities, the delthyrium closed by a pseudodeltidium ; 

 median portion of the valve not depressed in a sinus but sometimes 

 slightly flattened along an indefinite mesial band. 



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

 vexity posterior to the middle, the beak extending slightly beyond the 

 hinge-line posteriorly, the surface curving rather abruptly from the 

 umbonal region towards the cardinal extremities, but before reaching 

 the margin it is deflected to form a pair of small, nearly flat auriculations 

 a little more conspicuous than those of the opposite valve, forward from 

 the cardinal extremities along the lateral margins to the anterior margin 

 the curvature becomes gradually more gentle ; the median portion of the 

 valve is sometimes slightly flattened along an indefinite mesial band but 

 is not elevated in a fold. 



Both valves are marked by regular, simple, rounded, nearly uniform, 

 radiating plications which are so fine as to be almost indistinguishable 

 at the beak, but increase gradually in strength with the growth of the 

 shell. The number of these plications is usually between 42 and 48 

 on each valve. Lines of growth are inconspicuous or wanting. The shell 

 structure minutely punctate. 



Remarks. This species is closely allied to E. verneuilana and has some- 

 times been considered as a synonym of that species. There seem to be 

 some reasons, however, for its recognition under a distinct name, but 

 whether this designation should be considered as of specific or varietal 

 rank is largely a matter of personal opinion. There seems to the writer 

 to be little utility in using three names to distinguish a certain form, 

 and if it has characters sufficiently distinct to allow its being separated 

 from another, it seems worth while to designate it by a specific name. 

 Eumetria vera differs from E. verneuilana in attaining a larger size, 

 in having a somewhat smaller maximum number of plications, and in 

 the more anterior position of the greatest width of the shell. In addition 

 to these characters, no examples have been observed to attain so great a 

 convexity as is sometimes developed in E. verneuilana. 



Horizon, Chester group. 



EUMETRIA COST ATA (Hall) 

 Plate LXXVI, Figs. 25-29 



1858. Retzia, vera var. costata Hall, Geol. Iowa, vol. 1, pt. 2, p. 704, pi. 27, 



figs. 3a-b. 

 1894. Eumetria vera var. costata Hall and Clarke, Int. to Study of Brach., 



pt. 2, pi. 37, figs. 5, 11. 



