EUMETRIA 447 



to retain their separate names as applied by Hall, considering 

 costata as of equal rank with the others, however, and not as a variety 

 under E. vera. Each form possesses its individual characteristics and the 

 number of individuals possessing those characteristics is much greater than 

 those with intermediate characters. E. costata, perhaps, attains a larger 

 size than any of the others, but in general outline it is not essentially 

 different from E. vera, having its greatest width anterior to the middle 

 of the shell. The distinctive feature of the form is its coarser plications, 

 about 35 being nearly the normal number, while the other two species, 

 although usually smaller shells, have usually 45 or more plications. 

 Horizon. Chester group. 



EUMETRIA ACUTICOSTA n. sp. 

 Plate LXXVI, Figs. 30-34 



Description. Shell small, hinge-line very short, length and breadth 

 nearly equal, the greatest width near the mid-length of the shell, the 

 lateral margins diverging from the beak anteriorly nearly to the line 

 of greatest width of the shell, the anterior and antero-lateral margins 

 subsemicircular. The dimensions of a perfect specimen are : length of 

 pedicle valve 16.3 mm., length of brachial valve 14 mm., greatest width 

 16 mm., thickness 10.5 mm., length of hinge-line 4 mm. 



Pedicle valve most convex posterior to the middle, the umbo prominent 

 and projecting notably beyond the hinge-line, the surface curving from 

 the beak to the anterior margin with a regularly decreasing convex 

 curvature, towards the lateral margins the curvature becomes gradually 

 more abrupt in passing from the anterior margin posteriorly; towards 

 the cardinal extremities the surface becomes inflected and at the cardinal 

 extremities it is again deflected to form a pair of minute auriculations ; 

 the beak is prominent, incurved, and is pierced by a large, circular fora- 

 men which encroaches upon the umbonal region ; the cardinal area small, 

 arched, sharply denned, the cardinal margins sloping regularly from 

 the sides of the beak to the cardinal extremities; the valve is without 

 median sinus, the median portion not even being flattened as it is in 

 some members of the genus. 



Brachial valve subequally convex with the pedicle, the greatest con- 

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

 hinge-line, the surface curving rather abruptly from the umbonal region 

 towards the cardinal margin, the curvature becoming gradually more 

 gentle in passing anteriorly along the lateral margins to the anterior 

 extremity, along the median line of the valve from the beak anteriorly, 

 the surface describes a regularly diminishing convex curve, at the car- 

 dinal extremities the surface of the valve is deflected to form a pair of 

 minute auriculations. 



