i SILURIAN FAUNAS. 233 
flabellate, muscular impression, divided along its median line by a 
prominent median ridge, which reaches more than half way to the 
front of the shell. Surface marked by fine, radiating strie, of which 
about five occupy the space of two millemeters at the shell margin. 
The dimensions of an average specimen are: length, 11.5 mm.; 
width, 13 mm. 
Remarks.—This little shell is in all respects a diminutive form of 
the Helderbegian R. oblata H. In size it is similar to the Niagaran 
R. hybrida Sow., but differs from that species in usually being wider 
than long, instead of longer than wide. It also has a much larger 
museular impression in the pedicle valve than R. hybrida, im this re- 
spect resembling R. oblata. The prominent median ridge in the 
pedicle valve is also a character of R. oblata. The shell is in some 
respects intermediate between these two species, but approaches closest 
to the Helderbergian PF. oblata. 
PENTAMERUS CIRCULARIS 0. Sp. 
Plate XX., Figs. 17-19. 
Description.—Pedicle valve subcircular in outline, gibbous, the beak 
blunt, slightly incurved, cardinal area absent, delthyrium broadly tri- 
angular. Surface smooth, without mesial sinus, marked only by in- 
conspicuous lines of growth. Internally the valve has a strong median 
septum. The brachial valve is known only from fragments; it re- 
sembles the pedicle valve, but is less convex and lacks the prominent 
beak. 
The dimensions of the best-preserved specimen, a pedicle valve, are: 
length, 16 mm.: width, 16.5 mm.; convexity, 7.5 mm. 
Remarks.—The material representing this species is not as perfectly 
preserved as might be desired, but it seems to constitute an unde- 
scribed species. It most closely resembles P. ovalis, as identified by 
Foerste,* from Tennessee and Alabama, but is unlike the original P. 
ovalis of Hall, which is, perhaps, only a diminutive P. oblongus. 
* Proc. Bost. Soc. Nat. Hist., vol. XXIV., p. 324, pl. 5, figs. 17-18, and Geol. 
Surv. Ohio, vol. VII., pl. 30, figs. 17-18. 
