S. P. Woodward —Note on Plicatula sigillina. 113 
The largest specimen I have seen of this Plicatula measures 
8 lines by 10, the ‘breadth’ being usually greater than the 
‘height,’ and the elevation of the free margin never exceeds 2 
or 3 lines. A magnified representation of this shell is given in 
Pl. V. fig. 3; and a young Ostrea vesiculosa of similar size is 
added for the sake of comparison (Pl. V. fig. 6). It will be seen 
that their principal growth is in opposite directions, the Oyster 
fixing itself by its left valve, while Plicatula, like the rest of the 
Pectinide, is attached by the right shell. Fig. 1 represents a 
group of Plicatule, with Thecidium Wetherelli and other small 
shells, attached to a Helmet-urchin from the Upper Chalk of 
Kent. This beautiful and instructive fossil was presented to 
the National Collection by the Rev. Norman Glass, who at the 
same time expressed a wish that it should be described and 
figured. On another Helmet-urchin Mr. Glass detected a 
minute Plicatula with the upper valve preserved én siti, which, 
from its extreme tenuity, has taken an exact impression of the 
miliary granules on the surface of the Ananchytes below. In 
Dr. Bowerbank’s collection there is a similar example, but in 
the example figured (PI. V. fig. 5) the upper valve is imbricated 
with projecting laminze. 
The only known Plicatuda with which this shell can be com- 
pared is the P. inflata of James De C. Sowerby (Min. Con. 
pl. 409, fig. 2), a smooth, inflated species, not uncommonly 
found in the hard cream-coloured Chalk of Cambridge and 
Sussex, and the Grey Chalk of Dover. It is also met with in 
the Upper Greensand of Petersfield, the Isle of Wight, and 
Warminster. From these localities it has always been obtained 
in pairs, unattached; but it seems to have been fixed to extra- 
neous objects, at least when young, by a small portion of the 
umbo of the convex valve; the upper valve is deeply concave, 
as in Ostrea vesiculosa. Dr. Mantell described a young or dwarf 
condition of this shell, with slightly spinulose valves, by the 
name of Plicatulas pinosa (Geol. Sussex, pl. 26, figs. 16, 17). 
This variety approaches the P. pectinoides, J. Sby., found in the 
Gault of Folkestone, and in the phosphate-bed of Greensand at 
Cambridge, which is not deeper than a ‘ native’ Oyster, and the 
young individuals sometimes grow on the upper valve of the 
adult, notwithstanding its prominent spines. 
I am disposed to consider the Plicatula of the Upper Chalk 
as a distinct species from P. inflata, and to distinguish it by the 
name of sigillina, on account of its broad attachment and 
diminutive size. It is probably identical with the small Plica- 
tule which abound on the concretions, shells, and bones of the 
‘Coprolite-beds’ at Cambridge, and serve in a remarkable 
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