44 
SILURIAN 
somewhat variable in number and thickness, even consider¬ 
ably so,—a few being’ occasionally more prominent than the 
rest, and these reach the umbo. 
The area is flat, or a very little concave, and the deltidium 
(a) is obscure, but was evidently a narrow one, and probably 
covered over. 
Locality .—Damchen (16,500. Chorhoti Pass, 16,500 to 
17,000). (1678, 1754, 1737.) 
ORTHIS UNCATA. 
Plate 4, fig. 15. 
O. subglobularis ! unciam longa et lata (sicut visa oblonga), valva 
ventrali planiore, altera convexissima rotunda. Sinus ventralis in 
juveni nullus, dein profundus subacutus, in linguam longam productus. 
Sinus dorsalis nullus. Superficies tenuistriata, rugis concentrieis line- 
isque incrementi nullis. 
A very curious and unusual form of the genus, more glo¬ 
bular than even the gibbous varieties of the Orthis resujoinata 
in carboniferous rocks, and with a deep wide sinus produced 
into a long tongue, which, however, only indents the margin 
of the upper valve, and does not raise a convex fold on it. 
The whole shell is remarkably rounded and smoothed, and 
has far more the appearance of an Atrypa or Athyris than 
an Orthis. 
The ventral valve is only gently convex, with a short hinge 
line, and a small and very little prominent beak. It has a 
median depression running from the beak, which increases sud¬ 
denly in breadth and depth about half-way down the shell, and 
forms a broad tongue-like projection half an inch long in a 
full-grown shell. The borders of this sinus are rather abrupt¬ 
ly angular. Within, the dorsal lamellae are very short, and 
leave an angular space about 30° wide;—the muscular im¬ 
pressions are very slight. 
The other valve is very round and gibbous, with a small 
prominent beak not projecting beyond the short hinge line; 
