170 PALEOZOIC PALEONTOLOGY. 
and slightly incurved; the postero-cardinal region subalate. Basal 
and cardinal margins diverging posteriorly; the anterior end rather 
sharply rounded; the basal margin more or less sinuate at or a little 
in front of the middle of the shell, curving upward, both in front and 
behind, to meet the anterior and posterior margins; posterior margin 
broadly rounded below and obliquely truncate above. Umbonal ridge 
extending from the beak obliquely backward to the postero-basal .por- 
tion of the shell, sharply rounded or subangular near the beak, but 
becoming more broadly rounded posteriorly. In front of the umbonal 
ridge, extending from the beak obliquely backward to near the middle 
of the basal margin, is a rather broad and shallow sinus, which gives 
rise to the sinuosity of the basal margin. The most prominent point 
on the surface of the shell is upon the umbonal ridge at about the 
centre of the shell. Surface marked with somewhat irregular, con- 
centric lines of growth, which are usually more or less inconspicuous. 
The dimensions of average specimens are: length, 6 to 7 mm.3; 
height, 3.5 to 4 mm. 
Remarks.—This is one of the commoner species of pelecypods in 
the fauna of the higher beds of the Trenton formation, and it is 
believed to be identical with the little shell described from the New 
York Trenton limestone as Modtolopsis faba. The greater number of 
specimens preserved are right valves. Only a few left valves have been 
seen; they are slightly less convex, with a less conspicuous umbonal 
ridge. 
MODIOLOPSIS JERSEYENSIS N. Sp. 
Plate Soy Miz 9: 
Description.—Shell rather short and broad, subovate or subtri- 
angular in outline, anterior margip narrowly rounded, basal margin 
nearly straight through the greater part of its length, but curving 
upward in front and behind; posterior margin broadly rounded, 
curving regularly into the nearly-straight cardinal margin. Beak 
compressed, incurved, projecting a little above the hinge-line. Umbo 
flattened, umbonal ridge prominent, rounded, post-umbonal slope at 
first convex, becoming concave as it approaches the posterior cardinal 
extremity. The anterior slope from the umbonal ridge is occupied 
by a rather broad, shallow, ill-defined sinus, which extends from the 
