^2 Aug. F. Foerste 



anchylosed and forming a transverse ridge which is extended into 

 the umbonal ridge separating- the anterior truncated face from 

 the posterior part of the shell. This umbonal ridge is formed by 

 the most prominent plication on the shell. It is angular, the an- 

 terior face of the shell being deflected almost at right angles to the 

 posterior body. The anterior face extends as an obtusely angular 

 cone about one millimeter beyond the umbonal plane, and then is 

 produced as a laterally flattened acute extension along the hinge- 

 line for an additional distance of at least two millimeters, the 

 extreme tip not being preserved in the specimen at hand. Viewed 

 from in front, this extension appears like a vertical lamella, extend- 

 ing downward into the sharp angle formed by the junction of the 

 anterior margin of the valves. Anterior to the acutely angular 

 umbonal ridge is a low, broad plication, about one millimeter from 

 the umbonal ridge at the ventral margin. Anteriorly, this plication 

 is distinctly limited by a strong groove extending as far as the beak ; 

 posteriorly this plication is limited by another groove, distinct only 

 along the ventral half of the shell. Four or five practically obsolete 

 plications intervene between the prominent one just described and 

 the anterior extension of the shell. Posterior to the umbonal ridge, 

 the umbonal region is marked by two distinct plications, the first of 

 which is about one millimeter from the umbonal ridge, and the sec- 

 ond is about a third of a millimeter from the second. Deep grooves 

 intervene between the plications of this umbonal region, the anterior 

 groove being broad and conspicuous. Posterior to the second pli- 

 cation from the umbonal ridge, the shell is constricted distinctly, 

 and this posterior region is marked by distinct but narrow plica- 

 tions, of which twelve or thirteen reach the hinge-line in the speci- 

 men at hand. Several additional striae are intercalated anteriorly, 

 and the posterior border is imperfectly preserved. Conspicuous 

 striae, parallel to the ventral margin of the shell, cross the posterior 

 region of the shell, and produce a net-work with approximately 

 quadrate meshes. In the umbonal region these striae appear to be 

 almost obsolete. The anterior face of the shell is marked by a 

 net-work of extremely fine striae, part parallel to the plications 

 already described, and part parallel to the ventral margin of this 

 part of the shell. These striae are visible only under a strong 

 light carefully directed, when examined by a strong lens. The 

 umbonal ridge makes an angle of about 115° with the hinge-line. 

 The distance from the beaks to the ventral marciin of the shell at 



