158 PALEOZOIC PALEONTOLOGY. 
Remarks.—The internal characters of this shell have not been ob- 
served, so that its generic reference may be erroneous. Externally 
it resembles members of the genus Camarella more closely than any 
other shells which occur at this horizon. It differs from C. ambigua 
H. in the absence of the strong fold of the brachial valve and in the 
smaller sinus of the pedicle valve. From C. pandert Bill. it may be 
distinguished by its larger size and by the absence of the median 
plication in the sinus of the pedicle valve. 
PARASTROPHIA HEMIPLICATA (Hall). 
Plate X., Figs. 11-14. 
Description.—Shell subglobose, subpentagonal in outline, wider than 
long, the thickness frequently equal to the length. Cardinal line short, 
with sometimes the appearance of a small area on the pedicle valve. 
Each valve marked by from eight to twelve simple, subangular, radi- 
ating plications, which reach from one-third to one-half the distance 
from the margin to the beak, leaving the older portion of each valve 
smooth. Besides the radiating plications, the entire surface is marked 
by fine, concentric, subimbricating lines of growth, which are more 
conspicuous near the margin of the shell. Pedicle valve depressed- 
convex, with an abrupt, broad, but not deep sinus, which originates 
about one-third of the distance from the beak to the anterior margin, 
and is produced as a lingual extension in front at nearly a right angle 
to the plane of the valve; it is marked by from three to five radiating 
plications. The beak is small, closely incurved; delthyrium small and 
triangular. Brachial valve strongly convex or gibbous, with a broad 
mesial fold commencing one-third of the distance from the beak to 
the anterior margin, which is marked by from four to six radiating 
plications. 
The dimensions of a perfect individual are: length, 13 mm.; width, 
16.5 mm.; thickness, 12.5 mm. 
Remarks.—This species is characteristic of a zone near the base of 
the ‘Trenton, which may be the same as the Parastrophia zone recog- 
nized by White* in the region about Lake Champlain. 
* Bull. Geol. Soc. Am., vol. X., p. 459. 
—EE 
