OF SOUTnERN INDIA. 
17 
a margine fovaminis rapide declhi atque svlco prof undo ah apice valvcB Immalis 
separato ; laterihus nmhonalihtis roUnidate suh-aiugulal\s, pavlo con car Is ; area in 
idroque latere depresslKscidd, indistincte defmita ; peripheria vaharnm nndique 
simplici, fronte hand insinnata. 
This may justly be vcgartlccl as an offspring of the older depressa, nith ndiieh 
I would liaA'e identified it, were it not that all the speeimens (and there are several 
hundreds of them from various localities) from the higher beds entirely want the 
frontal sinus, and there is also no trace of an insinuation or overlap at the lateral 
margins, Avhieh is invariably more or less distinctly traceable in deqwessa. The more 
or less elongate or ovate form of both species is very similar and equally variable, 
the shells being often inclined to a symmetry, but in snh-deq)ressa the brachial or 
haemal valve is generally slightly less convex than the other. The margins are 
thin and acute in young, thickened or conspicuously truncate in very old ones. 
The surface generally shows distinct, sometimes slightly rugose, strife of growth, 
and besides a line, somewhat undulating or irregular radiating striation, similar to 
that of T. depressa, ohesa, disq)ariUs, and others. The beak in snh-depressa is al- 
ways more convex or incurved than in any speeimens of depressa I have examined, 
the deltidium is nuxch shorter than in the ordinary depressa, but agrees in that res- 
pect with var. egrta of the latter species, cxce])t that it is always very rapidly des- 
cending, and therefore separated from the apex of the hfemal valve by a deep 
sulcus. 
The loop must have been very short, almost shorter than in deqn'essa, the two 
hfcmal arms being at the beginning nearly parallel and very close to each other ; 
further on they diverge, but the terminal end has not been seen preserved (sec fig. 15 
on pi. ii). The sockets for hinge-teeth of the neural valve arc internally bounded 
by rather sharp and thin ridges. Below the loop on the inner side of the brachial 
or hfcmal valve there is a slight ridge, separating the two rather elongate chief 
muscular scars, at the outer side of whicb are tlie branched vascular inqiressions 
(see fig. IG on pi. ii). 
Localities. — Very common at Arrialoor, north of Poodoopolliam and at Imiloor ; 
rare near Clioekanadapooram, north and south-east of Ootacoil, everywhere either 
in a whitish calcareous sandstone, or a similarly colored impure limestone. 
Formation. — Arrialoor group, apparently a very characteristic species of this 
division. 
A peculiar variety, figured on ])]ate II, fig. 14, deserves special notice. It 
is from the Arrialoor sandstones, south-east of the town of Arrialoor, and is distin- 
guished from all the other specimens by a remarkably flattened and rounded form, 
the angle at the beak being above 100°. The deltidium appears to be very 
small, but the edge of the foramen is unfortunately not quite perfect. The surface 
is marked with sub-distant bands of growth and very dense n avy concentric lines, 
and is besides very densely punctated between them ; the margins of the valves 
arc in their entire circumference perfectly straight. 
E 
