234 PALEOZOIC PALEONTOLOGY. th: 
RHYNCHONELLA DECKERENSIS 0. Sp. 
Plate XXI., Figs. 1-4. 
Description.—Shell subtriangular, wider than long, the postero- 
lateral margins sloping from the beak, where they form an angle of — 
from 95° to 115°, in nearly straight hnes to a point a little posterior 
to the middle of the shell; the lateral and front margins regularly 
rounded. The pedicle valve is usually a little less convex than the op- 
posite one; its beak is prominent, arched, but not strongly incurved ; 
the sinus is rather abrupt, not reaching quite to the beak. The suface 
of the brachial valve curves gently to the margins, except toward. the 
front, where the mesial fold is rather abruptly elevated. The surface 
of each valve is marked by from twenty to twenty-four simple, an- 
gular plications, of which two or three, somewhat coarser than the 
remainder, are depressed in the median sinus, with a corresponding 
number elevated in the fold of the brachial valve. 
The dimensions of a rather large specimen are: length, 15 mm.; 
width, 19.5 mm.; thickness, 10 mm. 
Remarks.—This shell is a rather common one in the lower beds of 
the Decker Ferry formation. It resembles Rhynchotrema formosum 
Hall, from the higher portion of the Helderbergian series, but may be 
distinguished from that species by its coarser plications, its greater 
proportional width and by its less-strongly convex valves, which gives 
to members of this species a less thickness of the shell. The generic 
characters of the species have not been definitely determined, so that 
it is for the present referred to the genus Rhynchonella. 
RHYNCHONELLA AGGLOMERATA DN. sp. 
Plate XXI., Figs. 5-11. 
Description.—Shell subtriangular, usually a little wider than long, 
the valves subequally convex, the postero-lateral margins tapering to 
the beak, where they form an angle of about 90°; the lateral and 
anterior margins rounded. Pedicle valve most prominent near the 
umbo, the beak sharply pointed, arched over that of the opposite valve ; 
mesial sinus rather shallow, rounded in the bottom, not extending 
