ORDOVICIAN FAUNAS. 209 
of the valve as two is to three, the two extremities almost equally 
angular; height of ends about as three is to four, both obliquely 
truncate above, the anterior narrowly rounded in the middle; the 
posterior outline more broadly and evenly curved, though having 
the usual backward swing. Ventral edge of carapace obtuse, scarcely 
flattened, with a slight furrow on each side near the edge of the right 
valve, in which a row of minute puncte is generally distinguishable ; 
overlap extending all around the free edges, strongest ventrally; ex- 
cept in rare instances neither valve has a flange or flattened border, 
and when present, it is in all cases very narrow and undefined; dorsal 
edge somewhat thickened, especially upon the left side. Surface of 
valves smooth or very faintly pitted, rather evenly convex, with the 
greatest thickness somewhat beneath the centre; a low ridge-like 
thickening along the posterior half of the dorsal margin of the left 
valve is to be noticed. Eye tubercle just distinguishable in most 
cases, often not to be detected. On the inner surface, however, it is 
always marked by a distinct pit. Muscle spot not distinguishable 
externally, except when the specimens are weathered, but on the inner 
side it is often well marked and surrounded by fine, reticulating, 
radial lines, short dorsally, longest post-ventrally. On the inner 
side of the ventral edge of the right valve there are two rows of small 
papille, three to five each, the number seeming to increase with age. 
The purpose of the papille, one series of which occurs in the anterior 
third, the other in the posterior, evidently was to prevent undue over- 
lapping of the valves by presenting an obstacle to the entering ventral 
edge of the left valve.”—UIrich. 
LEPERDITELLA ORNATA Nl. Sp. 
Plate XIII., Figs. 13-15. 
Carapace subelliptical; length of a large specimen, 3 mm.; height, 
2 mm.; thickness, 1.7 mm.; length of hinge line, 1.7 mm. Valves 
equally convex, greatest thickness below the middle. Free margin 
of the right valve abruptly inflexed, with a row of minute tubercles 
upon the angular, marginal ridge thus formed. Free margin of the 
left valve incurved and covering the inflexed margin of the opposite 
valve, with an external, longitudinal groove near the ventral margin. 
14 
