ORDOVICIAN FAUNAS. alyyial 
beak obliquely backward to the basal margin. Anterior extremity of 
the shell compressed. The most prominent point on the shell is near 
the centre, on the umbonal ridge. Surface marked by more or less 
obscure, concentric lines of growth. 
The dimensions of a perfect cast of the right valve are: length, 15 
mm.; width, 10 mm.; convexity, 3 mm. 
Remarks.—This species may be recognized by its rather short and 
broad form, the dattened umbo and the oblique sinus. 
MODIOLOPSIS DEPRESSA Nn. Sp. 
Plate Xi. Fig. 8: 
Description.—Shell subovate in outhne, the valves depressed-con- 
vex. Anterior margin regularly rounded from beneath the beak into 
the nearly-straight or shghtly-convex basal margin; the posterior 
margin broadly and regularly rounded. Beak small, a little elevated 
above the hinge-line; umbonal ridge broadly rounded and ill-defined ; 
the post-umbonal slope convex above, becoming concave toward the 
hinge-line; antero-basal slope with a rather broad, shallow, ill-defined 
depression or sinus extending from the basal margin half way to the 
beak. Surface marked by obscure, concentric lines of growth. 
The dimensions of a nearly-complete right valve are: length, 16 
mm.; width, 10.5 mm.; convexity, 1.5 mm. 
Remarks.—This species may be recognized by its depressed-convex, 
nearly-smooth valves, with obscure umbonal ridge and small beak. 
ORTHODESMA CANALICULATUM Ulrich. 
Plate. XU., Fig. 5. 
189%. Orthodesma canaliculatum Ulr., Pal. Minn., pt. II., p. 520, 
pl. 3%, figs. 7-11. 
Description.—‘Shell elongate, the length three times the height; 
cardinal and basal margins straight, nearly parallel; posterior mar- 
gin oblique, rounding into the hinge-line, below which it slopes back- 
