232 PALEOZOIC PALEONTOLOGY. nae 
The dimensions of the best specimen observed are: length, 17 mm. ; 
width, 22 mim. 
Remarks.—The only specimens of this shell which have come under 
observation are more or less imperfect casts of the interior, and only 
brachial valves have been seen. These cannot be certainly identified 
with O. flabellites Foerste, but they are closely allied to that species, 
and it is probable that they should be included in it. 
DALMANELLA POSTELEGANTULA Ni. Sp. 
Platé XX., Figs. 21-24. 
Description.—Shel} subcircular in outline, usually wider than long, 
the hinge-line shorter than the greatest breadth. Pedicle valve 
strongly convex, subcarinate, the beak incurved over the cardinal area. 
Brachial valve depressed convex with a shallow, rounded mesial sinus 
which does not reach to the beak. Surface of both valves marked with 
fine radiating strie, from seven to nine of which occupy the space of 
2 mm. at the shell margin. 
The dimensions of an average specimen are: length, 10.5 mm.; 
width, 12 mm. 
Remarks.—This species is intermediate in its characters between 
the Silurian D. elegantula and Helderbergian D. subcarinata, and 
in a previous report was identified with D. clegantula. It differs 
from D. elegantula, however, in being usually wider than long instead 
of longer than wide, and in having a more convex brachial valve, with 
a much more conspicuous mesial sinus. In these same characters it 
approaches the Helderbergian species D. subcarinata, but it does not. 
attain so large a size, and the sinus in the brachial valve does not 
extend to the beak, as in that species. 
RHIPIDOMELLA PREOBLATA Ni. Sp. 
Plate XX., Figs. 25-26. 
Description.—Shell lenticular in form, subcircular to subelliptical 
in outline, usually a little wider than long. the hinge-line a little over 
one-half the greatest width of the shell. Pedicle valve with a large, 
