304 PALEOZOIC PALEONTOLOGY. 
RHIPIDOMELLA OBLATA (Hall). 
Plate XXXYV., Figs. 9-13. 
' See, also, p. 350. 
1859. Orthis oblata Hall, Pal N.Y., vol. LIL... p. 162.) piSaiheaee: 
1-22. 
Description.—Shell sublenticular in form, transversely subelliptical 
in outline when adult, the hinge-line a little less than one-half the 
total width. Pedicle valve convex at the beak and on the umbo, 
flattened in the middle and slightly concave toward the lateral and 
front margins; the beak pointed, produced but slightly beyond that 
of the brachial valve; cardinal area small, concave. Brachial valve 
convex, deeper than the pedicle valve, with a slight mesial flattening, 
extending from the beak to the front margin; the beak nearly even 
with that of the opposite valve. Surface marked by fine, raised, 
rounded strizw, which increase by frequent bifurcation, and are curved 
toward the lateral and cardinal margins. The strizw are crossed by 
finer concentric striz and by stronger lines of growth, which become 
more numerous and crowded towards the margin of the older shells. 
The dimensions of a nearly-perfect specimen of average adult size 
are: length, 28 mm.; width, 31 mm.; thickness, 12 mm. 
RHIPIDOMELLA EMINENS (Hall). 
Plate XXXIV., Fig. 9. 
1859. Orthis eminens Hall, Pal. N. Y., vol. III., p. 167, pl. 11, figs. 
t—-14. 
Description—Shell subcircular in outline, usually a little wider 
than long; hinge-line about two-thirds the greatest width, cardinal 
angles obtusely angular. Pedicle valve strongly convex posteriorly, 
the umbo prominent, the surface sloping rather steeply to the cardinal 
border, flattened or depressed anteriorly ; beak prominent, slightly in- 
curved, projecting beyond that of the opposite valve; cardinal area 
rather large, moderately concave; the delthyrium broadly triangular. 
