SILURIAN 
34 
STROPHOMENA TRACHEALIS. 
Plate 3, fig*. 7. 
S. tenuis parva, vix semiuncialis, fronte (non adhuc invento) 
deflexo ; concentrice perrugata, rugis 7, radiatimque sti'iata. Strife 
tenuissimae, nonnullis remotiusculis exceptis. 
Shell very thin, half an inch wide,, and rather more than 
half that length, with concentric regular furrows, and finely 
radiated with close striae,—every tenth or twelfth of which is 
much stronger than the rest: rostral portion (all we yet know) 
concentrically rugate, with about seven equal-sized ridges, 
which turn but little outwards at the hinge margin. Ventral 
valve scarcely convex (almost flat) at the beak. Opposite 
valve regularly concave, with a minute projection at the beak. 
It is possible that the valves are reversed in the above des¬ 
cription, and the gently-convex valve with its minute rostral 
projection may be the ventral or enveloping valve. It is im¬ 
possible to decide, unless the deflected portion of the valve 
were present. For the same reason we can hardly be sure of 
the specific distinction between this and the S. tenuistriata, of 
the British Caradoc rocks. But as all the otherf ossils are 
distinct from European ones, it is reasonable to suppose this 
flat-valved shell, with very regular rugae, a distinct form. 
The species just quoted is rather irregular in its rugae, and is 
besides a far larger shell. 
Locality .—Chorhoti Pass, 17,000-17,500 feet. (Nos. 1737, 
1738, 1743.) 
STROPHOMENA.— Sp. (like S. depressa, Dalm.) 
A shell allied to the above, but with much coarser nurse, 
has a front precisely in the way of the S. dejjressa, a very 
common shell in all the slate rocks of Europe. It has six or 
seven broad rugae, and the rostral portion is no broader than 
the deflected front margin. 
Locality .—It occurs in various localities : Milam Glacier; 
Bompras ; Chorhoti; Rimkin. 
