SILURIAN FAUNAS. 
ra) 
at 
Sy 
anterior margin narrower and often more sharply rounded than the 
posterior. Hye tubercle small, but rather prominent, situated at a 
point about one-third the length of the straight dorsal margin from 
its anterior end and about three-fourths of the height of the valve at 
that point from the basal margin. Surface of the valves flattened 
at the antero-dorsal angle, the flattening extending as a narrow, indis- 
tinct, marginal border nearly to the centre of the anterior margin. 
Along the posterior margin there is a similar, narrow, indistinct, 
flattened border, which extends from the postero-dorsal angle to a 
point in the ventral margin directly below. Surface of valves per- 
fectly smooth. 
The dimensions of an average specimen are: length, 7 mm.; height, 
4.5 mm.; convexity of one valve, 1.5 mm. 
Remarks.—This species most closely resembles L. alta of the Man- 
lius limestone, but may be always distinguished from that species by 
the indistinct, posterior marginal border. It also resembles L. jonest, 
but does not have the pitted surface of that species. 
BEYRICHIA SUSSEXENSIS Nl. Sp. 
Plate XXIII., Figs. 3-4. 
Description.—Carapace subelliptical in outlne; the dorsal margin 
straight, its length about nine-tenths of the total length of the valves, 
its extremities angular. Ventral margin gently convex; the anterior 
and posterior margins both broadly rounded, the posterior extremity 
being a little higher than the anterior and its margin being nearly 
straight for a short distance below the postero-dorsal angle. The 
entire free margin of the valves with a distinct, flattened, marginal 
border, which is abruptly separated from the body of the valve. The 
dorsal half of the surface of each valve is divided into three lobes by 
two deep furrows, which reach to about the middle of the valve. ‘ihe 
posterior furrow divides the valve at about the middle of its length 
and is nearly straight; the anterior furrow lies about half way be- 
tween the posterior one and the anterior extremity; it is a little 
shorter than the other and its lower extremity is slightly bent back- 
ward. ‘The two furrows are connected below by an indistinct de- 
pression in the valve, partially isolating the middle lobe. The anterior 
and posterior lobes are connected ventrally. The middle lobe is.sub- 
