THE KIMMSWICK AND PLATTIN LIMESTONES 
203 
pedicel valve is moderate, scarcely exceeding 2 mm. ; the convex- 
ity of the brachial valve may not have exceeded 7 mm., as far 
as may be judged from the part remaining. The hinge-area of 
the pedicel valve is relatively high, equalling at least 3 mm. at 
the beak. The angle between this cardinal area and the general 
surface of the pedicel valve is about 15 degrees. The postero- 
lateral angles equal about 70 degrees. The surface of the valves 
is covered with radiating striae, 6 or 7 in a width of 2 mm. Every 
fourth one of these striae tends to be slightly more prominent, 
this tendency being greater in the brachial valve. The individ- 
ual specimen here described is associated with numerous speci- 
mens of typical Strophomena incurvata, and may be only an 
aberrant form of the latter. 
14. Trematis huronensis Billings 
Outline circular, upper valve moderately convex, the convexity 
increasing toward the beak. Largest specimen found is 25 mm. 
in width and 22 mm. long. The surface is ornamented by radi- 
ating and concentric striae, the intercepted areas being occupied 
by small elliptical or more or less quadrangular pits. In some 
specimens the striae and pits are distinctly perceptible as far as 
the beak. In these specimens, both the radiating and the con- 
centric striae tend to be narrow, and the intercepted pits tend 
to be more quadrangular. In other specimens the pits diminish 
more rapidly in size than the intermediate striae toward the 
beak, finally appearing as minute circular pits arranged in rows 
on an area which otherwise is smooth; sometimes the pits dis- 
appear altogether before reaching the beak. In a third group 
of specimens, the concentric striae are considerably narrower 
than the radiating ones, at least posteriorly, and especially be- 
tween 5 and 10 mm. from the beak. Anteriorly, in the more 
mature stages of growth, the specimens are much alike; both the 
radiating and concentric striae are narrow and the intercepted 
pits are more or less quadrangular. In the mature stages of 
growth the number of pits usually varies from 8 to 9 in a width of 
2 mm., but in the final stages of growth this number may in- 
