FAUNA OF THE TRENTON GROUP. 
23 
Rafinesquina deltoidea, which occurs in the same beds and even on 
the same slabs with it, and without good specimens it is difficult to 
distinguish the two, even though they belong to different genera. 
Description. Shell roughly triangular, nasute in front, widest 
at the hinge. Brachial valve from moderately to very strongly convex, 
with a sharp geniculation near the middle of the valve. At the beak 
there is a flattened area, which is sometimes even a little concave. 
The pedicle valve is convex at the beak, becoming deeply concave in 
front of the umbo. The surface of both shells is covered with fine 
striae of two sizes. The stronger are rather numerons in most shells, 
and have groups of four to six fine ones between them. The posterior 
margin and that half of the shell are in most cases covered with 
fine, concentric wrinkles like those of Rafinesquina deltoidea . Rather 
numerous interiors of the pedicle valve of this species have been 
found at Division Btreet, Ottawa, and one or two at other localities. 
The muscle-scar is small and rounded, almost circular, with the usual 
gap in the rather thin rim at the front; at the gap the rim turns 
back in two converging lines to the beak. There is no trace of any 
median ridge or septum. Outside the scar, the surface is marked by 
the radiating lines of the stria? of the exterior. 
Horizon and Locality. Upper part of the Trenton (Upper 
Cobourg), at Picton and other places in Prince Edward county, at 
Collingwood, and at Ottawa. In Minnesota it occurs in the Prosser 
limestone (Fusispira beds) in association with Rafinesquina deltoidea. 
In Ontario,, this species becomes very abundant and seems to replace 
R. deltoidea in the upper beds of the Cobourg. 
Strophomena foveata sp. nov. 
Plate Vli, figure 4. 
Shell small for the genus, wider than long, the brachial valve 
gently and uniformly convex except for a flattening at the beak, and 
a slight sinus which extends about half-way to the front. An obscure, 
broad depression, just below the cardinal extremities, produces a 
turned-up appearance. Pedicle valve convex at the nmbo and evenly 
concave in front. Cardinal area of the pedicle valve rather narrow, 
deltidium broad and low, but convex. Pedicle opening quite large, 
encroaching upon the beak. Both valves covered with fine, thread- 
like striae which increase by bifurcation, and are somewhat irregular 
in appearance. Every other striation is, in general, weaker than its 
neighbour. Very numerous and fine concentric markings cross the 
radial striae and in places produce small pits which add to the some- 
what rough appearance of the surface. 
