ORDOVICIAN FAUNAS. 163 
beak to the antero-basal angle; between it and the edge of the lunule 
a narrow sulcus; behind it a small, well-marked, mesial sulcus, out of 
which the surface rises more gradually to the summit of the posterior 
umbonal ridge. The most prominent point of the surface of the 
valves is situated on this ridge, somewhat above the middle of the 
height and about two-fifths of the length from the anterior extremity. 
Surface marked with nearly equal, concentric undulations or ridges. 
These are strongest in the mesial sulcus, somewhat flattened, yet 
distinct, in the anterior sulcus, and nearly obsolete on the cardinal 
slope. Hinge and muscular impressions undetermined.”—UlTrich. 
Remarks.—The most perfect individual of this species which has 
been observed from the Trenton limestone of New Jersey differs from 
the illustration of the type of the species in having the posterior cardi- 
nal portion of the shell less alate. In all other respects the agree- 
ment between the two shells is very close, and there seems to be no 
justification in considering them as specifically distinct. 
CTENODONTA NASUTA (Hall). 
Plate X1., Fig. 1. 
1897. Ctenodonta nasuta Ulr., Pal. Minn., pt. II., p. 584, pl. 42, 
iE) 
Description.—*“‘Shells transversely elongate, subovate, the length 
one-twentieth or more greater than twice the greatest height; beaks 
rather small, not very prominent, incurved, situated about one-twelfth 
of the entire length in front of the middle; anterior end large, broadly 
and regularly rounded; posterior end produced, tapering, rather nar- 
rowly rounded at the extremity; cardinal margin nearly straight, 
basal line broadly convex, except for some distance behind the middle, 
where it is straight or, more often, gently sinuate. Greatest thickness 
near the middle of the anterior half, equaling about one-third of the 
length of the shell. Umbones moderately inflated, the posterior cardi- 
nal slope defined by an obscure umbonal ridge, very abrupt for a short 
distance behind the beaks, more so than on the anterior side; a broad 
and very shallow sulcus crosses the valves obliquely from the umbones 
to the contraction in the base. Ligament attached to a sharply-de- 
fined groove on each side of the hinge-line, extending from the beaks 
