38 
SILURIAN 
shape,, slightly convex in the ventral and flat in the dorsal 
valve, radiated with fine ribs, of which the central one is by 
far the strongest. A small narrow area toward the beak of 
the convex valve is elevated, otherwise the valve is regularly 
and gently curved. The ribs or stronger striae arch a little 
upwards, interlined by several striae, which are fine and cross¬ 
ed by conspicuous ridges of growth in most of the varieties. 
Along the hinge margin there are rugae directed outwards. 
The central rib is very strong, dividing the shell into two ap¬ 
parent halves. And all the ribs, both in the plain and corru¬ 
gated varieties, are nodular or tubercular. Under a lens, 
minute transverse striae are visible on all the ridges. 
The flat valve (not figured in our plate) differs in the want 
of the large central ridge and the conspicuous principal ones. 
The ribs are equally nodose. There is a minute central pro¬ 
minence on the beak of this valve, which appears to be a cons¬ 
tant character. 
Locality .—Milam ; Bompras; Chorlioti; Kalajowar. (Nos. 
1662, 16Go, 932, 1737-8, 1757.) 
STROPHOMENA HALO. 
Plate 3, fig. 13. 
S. oblonga, unciam lata, 7 lineas longa, angulis rectis, valva veu- 
trali ad natem convcxa, deinde plana, concentrice subundulaat. Margo 
cardinalis rugosus. Stria3 radiales confertee, minuta3, sequales, lineisque 
transversis confertissimis scabrte. 
One inch wide, seven lines long, rather more than semicir¬ 
cular ; ears not produced; ventral valve convex at the beak, 
then nearly flat, obscurely undulated, rugate along the hinge 
line, and radiated by fine numerous equal striae crossing the 
irregular undulations of the surface; under a lens they are 
marked by fine close sharp transverse threads. 
A pretty species, with the surface concentrically rugose, 
but only in parts. Along the hinge line there are several very 
oblique transverse plates. These are stronger than the rugae 
of the disk, which are quite irregular and not very well 
defined. The front may have been turned down as in 
