338 
Aug. F. Foerste 
description of Modiolopsis sulcata is greatly overdrawn. Not only 
the low transverse plications along the basal margin but also those 
along the posterior part of the cardinal margin can be detected. 
Judging from other specimens of Pholadomorpha pholadiformis, 
one of the faint, broad, radiating grooves of the postumbonal area 
usually is located immediately above the crest of the umbonal 
ridge, and a second groove is found a short distance above the 
first. Additional grooves may occur nearer the hinge-line. 
45. Rhytimya cymbula, Miller and Faber 
(Plate VI, Figs. 8 A, B) 
189 1^. Orthodesma cymhula Miller and Faber, Jour. Cincinnati Soc. Nat. Hist., 
1 7, p. 1J^3, Plate 8, Figs. 7-9 
The type of Orthodesma cymhula is numbered 8814 in the Faber 
collection at Chicago University. It was described as not very 
uncommon in the upper part of the Cincinnati series of rocks in 
Warren county, Ohio, but the type is labelled as coming from 
Cincinnati, Ohio. This type, undoubtedly, is identical specifically 
with Rhytimya mickleboroughi Whitfield, as figured by Ulrich in 
the Geology of Ohio, volume 7, from the Fairmount member of the 
Maysville group, a fact already indicated by Bassler in his Biblio- 
graphic Index of American Ordovician and Silurian Fossils. 
The shell substance is very thin. Owing to the prominent 
umbonal ridges and the flattened dorsal slopes, the cross-section of 
the shell is strongly triangular. The dorsal flattening is conspicuous 
for a distance of about 35 mm. from the beaks, and the umbonal 
ridge makes an angle of about 15 degrees with the hinge-line. A 
distinct mesial depression extends from the beaks toward the middle 
of the ventral margin, broadening toward the latter, and forming a 
concave outline along the latter. Anterior to this depression the 
shell is swollen along a line forming an angle of about 70 degrees 
with the hinge-line. Between this part of the shell and the um- 
bonal ridge there are obscure traces of radiating striations. The 
anterior part of the shell is almost acutely pointed. The upper 
posterior part of the shell, and the tip of the anterior part are not 
preserved. The characteristic wrinkles of Rhytimya are strongly 
developed anteriorly, are much less conspicuous within the area 
of the broad mesial depression, and are weakly defined on the post- 
umbonal slopes. 
