40 
SILURIAN 
except the actual beak, which is convex. Hinge line not 
more than the width of the sukquadrate shell. 
The above may be considered as the general characters of 
this shell, which varies as greatly as the 0. flabellulum of the 
British Caradoc rocks, and appears to be the exact represen¬ 
tative of it. Like that common shell, it has the median line 
in the convex dorsal valve slightly depressed, and the ventral 
valve is flat. 
The typical forms of both species have few simple rounded 
ribs, and the extreme varieties have them very numerous and 
much branched, and the size is about the same. 
I shall now describe the varieties, beginning with the 
simple-ribbed ones. The specific name is suggested by a 
Himalayan palm, the due nicer ops Marshiana (TliaTcil ). 
Var . convexa. Yigs. 8, 9. 
—Major, costis 14 simplicibus rotundatis. 
About fourteen lines broad and nine lines long, subquad¬ 
rate : the dorsal valve very convex, its ribs, about fourteen, 
of which the four central ones form a somewhat raised central 
space, without any marked median depression. Ribs rounded, 
about their own diameter apart, and with no intermediate 
ones. 
Locality .—Milam Glacier (931), Damchen ; Kalajowar. 
Var. /3-TRIFIDA. F ig. 10. 
—Planior ? costis angustis trifidis, 
« 
In this variety, which is smaller (and flatter ?) than the last, 
the ribs are not so broad, but are angular and divided for 
rather more than half their length into threes : the secondary or 
lateral ribs being less prominent than the principal ones. 
Locality .—Niti Pass (1782, 1766, 1737, 1754); Upper 
Rimkin; Chorhoti; Kalajowar. 
