202 
GEOLOGY OF THE FOURTH DISTRICT. 
The raised lines are less arched than in L. sinuosa (Terebra sinuosa, Silurian Researches, 
pi. 8, fig. 15), which also occurs in this group, and are almost straight on some of the higher 
whorls. 
Locality —Seneca lake ; Moscow. 
The specimens figured in the following illustrations appear to be confined to the thin band 
of Encrinal limestone below the Moscow shale. 
1. Avicula orbiculata, n. s. — Roundish, auriculated; slightly oblique hinge line shorter 
than the width of the shell; posterior wing extending nearly in a line with the margin of the 
shell. Surface covered with rounded radiating ribs, which become fainter upon the upper 
lateral margins ; these are crossed by rounded concentric lines, which are less prominent than 
the others. 
The nearly round figure of this shell forms a prominent feature of its distinctive character. 
Locality —Eighteen-mile creek. 
2. Atrypa rostrata, n. s. — Obovate ; valves nearly equally convex, lower one with a lon¬ 
gitudinal depression from beak to base ; beak of the upper valve scarcely prominent, of the 
lower valve very prominent, produced and incurved over the upper one. 
This is a very neat little shell, marked by a few concentric lines of growth. 
Locality —Eighteen-mile creek. 
3. Atrypa -, n. s. — Elliptical, slightly elongated at the beak ; valves nearly equally 
convex, smooth, or with faint concentric lines of growth ; beak of the lower valve prominent, 
rather small, and slightly incurved. 
This shell is larger than the last, the beak smaller proportionally, and less incurved, and 
there is no depression on the lower valve. 
Locality —Eighteen-mile creek. 
