SILURIAN FAUNAS. 267 
right angle, curving below into the convex, ventral margin; anterior 
margin broadly rounded, meeting the hinge-line in an obtuse angle. 
Free margin with a narrow, flattened border. Valve divided into three 
lobes by two vertical furrows from the dorsal margin; the posterior 
furrow extending nearly to the middle of the valve, its lower extremity 
slightly curved forward; anterior furrow less deeply impressed, only 
about one-half as long as the posterior one. Posterior lobe occupying 
nearly one-half the total length of the valve; middle lobe subglobular, 
about as high as the posterior one; anterior lobe sloping away to the 
front. Surface marked by fine, closely-arranged, circular pits, except 
in the furrows and on the marginal border. 
The dimensions of the type specimen are: length, 1:9 mm.; height, 
1 mm. 
BEYRICHIA MONTAGUENSIS Nl. Sp. 
Plate XXIV., Fig. 23. 
Description.—Carapace suboval in outline, narrower in front, the 
hinge-line straight, about five-sixths of the entire length of the valve. 
Postero-dorsal angle a little produced, so that the posterior margin is 
slightly concave above. Free margin with a rather broad, flat border. 
Valves divided into three lobes by two furrows, which extend vertically 
downward from the dorsal margin; the posterior furrow the longer, 
reaching a little below the middle of the valve, its lower portion 
slightly curved toward the front; anterior furrow scarcely reaching 
the middle of the valve, slightly curved, with its concave side pos- 
terior, a little narrower and less deeply impressed than the posterior 
one. Posterior lobe convex, occupying about one-half of the length 
of the valve; middle lobe the most prominent, subglobose; anterior 
lobe the lowest of the three, its surface sloping to the anterior border. 
Surface of the valve minutely pitted or punctate, except in the fur- 
rows and on the marginal border. 
The dimensions of the type specimen are: length, 2.4 mm.; height, 
1.6 mm. 
