32 
SILURIAN 
the rest of the shell, so as to equal the length of the valve. 
And the ears, in those specimens which are of an oblong 
shape, are a little produced. In others of a rounder shape, they 
are continuous with the shelving sides, which slope away steep¬ 
ly from the central or gibbous portion. This part is occasion¬ 
ally somewhat flattish, especially near the broad blunt umbo, 
but is generally regularly convex. 
The dorsal valve is deeply concave, and much shorter and 
proportionally wider than the larger valve (decidedly transverse 
in the more .semi-oval (fig. e) forms). 
This valve is specially concave under the beak ; it often pre¬ 
sents an abruptly-depressed nucleus, which lies at right angles 
to the rest of the valve (this is a raised space of course, on the 
cast, cl ). It is also at right angles to the area of this valve, 
which is broad and parallel-sided, and has a covered convex 
deltidium as in other species. Both valves are closely ribbed 
by equi-distant thread-like strife regularly interlined and in¬ 
creasing in number, and crossed at intervals by strong lines 
of growth. 
This little species has the strongest analogy with the British 
species, L. teimissime-striata of the Caradoc rocks, and with 
the L. oblonga of Russia. 
Locality .—Kalajowar (15,800 feet) ; Milam Glacier. (Nos. 
933, 1764, 1766.) 
STEOPHOMENA- Rafinesque; King; Davidson; Sfc. 
The distinctions of these three genera, Leptcena, Strop ho- 
mena , Orthis, which occur in every Silurian formation, are no 
less easy to determine by the external characters (with some 
exceptions, as in the case of the shell last described) than by 
a study of the interior structure. 
L&ptccna, is always an involute shell; the valves curved 
strongly, and wrapping the one round the other, so that one 
shall be very convex, the other greatly concave. With this 
character are joined the internal ones of elongated muscular 
impressions, and the lateral teeth of the hinge being confound¬ 
ed with the central boss in the dorsal (or concave) valve. 
