SILURIAN FAUNAS. 263 
rounded or slightly-flattened plications on each side of the fold and 
sinus and by conspicuous, more.or less crowded, concentric lines, which 
are arched over the plications. Besides these markings, very fine, 
radiating striz may be detected on well-preserved specimens by the aid 
of a magnifying glass. 
The dimensions of a rather large pedicle valve are: length, 10 mm. ; 
width, 11.5 mm.; convexity, 4.5 mm. Those of a brachial valve are: 
length: 7.5 mm.; width, 11 mm.; convexity, 2.5 mm. 
Remarks.—This is one of the most characteristic species of the Man- 
lius limestone fauna, and occurs throughout the formation, being 
especially abundant in some of the beds. 
MOLLUSCA. 
PELECYPODA. 
MEGAMBONIA AVICULOIDEA Hall. 
Plate XXIV., Figs. 17-18. 
1859. Megambonia aviculoidea Hall, Pal. N. Y., vol. IIL., p. 274, pl. 
49, figs. 7 a—b, pl. 49 A, fig. 8. 
Description.—Shell oblique, winged posteriorly. Beaks nearly ter- 
minal, prominent, obtuse, incurved, elevated above the hinge-line. 
Hinge-line straight; anterior margin curving forward from beneath 
the beaks for a short distance, then curving backward, with a long, 
gentle sweep, to the postero-basal extremity, which is more or less 
broadly rounded; posterior margin sinuate below the hinge extremity, 
convex towards the base. Surface of valves strongly convex on the 
umbonal ridge; the anterior slope rather abrupt, slightly convex; pos- 
terior slope concave, the posterior wing not sharply separated from 
the body of the shell. Surface marked by fine, more or less unequal, 
concentric lines of growth, which are uniform in character upon the 
body of the shell and the posterior wing. 
The approximate dimensions of an imperfect specimen are: height, 
16 mm.:; length, 16 mm. 
