FAUNA OF THE TRENTON GROUP. 
19 
in the region of Cincinnati, specimens which would correspond very 
closely with P. ammna. 
Horizon and Locality. The species as here defined is found 
throughout the Trenton from the hase to the top, but is not especially 
common except at certain horizons. At Trenton Falls it is abundant 
at two horizons, one 175 and the other 240 feet above the base. The 
lower zone is at the top of the Prasopora beds and the upper one in 
the Rafinesquina deltoidea beds.” In Canada it is not especially 
abundant anywhere, though common enough at various places. 
Platystrophia champlainensis MeEwan, 
Plate VI, Figures 6-9. 
Platystrophia trentonemis champlainensis MeEwan, Proc. II. S. Nat. 
Mus., vol. 56, 1919, p. 407, PI. 42, figs. 9-11. 
Cumings, in his brilliant paper on the u Morphogenesis of 
Platystrophia ” pointed out that in the common American Ordovician 
Platystrophias, the young shell had first one plication in the sinus, 
and that new ones were added by implantation at the sides, whereas 
in European species of the same genus, the median plication bifur- 
cated, so that, fundamentally, there are three plications in the sinus 
of the American forms, and two in the European. He referred, 
however, to the fact that in the basal Trenton of the Champlain 
valley, there is a small form which follows the European rather than 
the American method in the introduction of plications on the fold 
and sinus. 1 This small shell is fairly common at Montreal in 
the lowest layer of the Trenton and as one of the oldest, if not the 
oldest, of American Platystrophias, it is deserving of a specific name. 
Description. Shell small, the valves subequally convex, with a 
shallow sinus and low, flat- topped fold. The hinge line is considerably 
less than the greatest width, the cardinal angles projecting but 
slightly. Cardinal areas narrow, the beaks incurved, that of the 
pedicle valve extending slightly beyond the other. On all specimens 
so far seen the sinus contains three plications and the fold four. The 
outer pair of plications in the sinus are produced by the bifurcation 
of the original single plication, and the third one is implanted between 
them. The median pair on the fold arise from a single one near the 
beak, the outer pair being the original plications. On the sides there 
are seven to eight ribs on the brachial valve, and eight to nine on the 
pedicle valve. The largest specimen is 11 mm. long and 14 mm. 
wide; the smallest, 6 mm. long and 7 mm. wide; the holotype, 9 mm. 
long, 13 mm. wide, and 8 mm. thick. 
i Am. Jour. Sc., vol. XV, 1905, p. 9 and p. 41. 
10711 — 3$ 
