12 S. P. Woodward—WNote on Plicatula sigillina. 
IV. Nore on PrrcaTuLa sIGILLINA, AN UNDESCRIBED FOSssIL OF THE 
Uprer CHALK AND CamBrinGr PHOsPHATE-BED. [Plate V. 
figs. 1-6. | 
By 8S. P. Woopwarp, F.G.8., A.L.S., &e. 
VERYONE who has collected the fossils of the Upper 
Chalk will have noticed how the Echinoderms and Belem- 
nites are overgrown by small oyster-like shells with a striated 
disk, commonly passed by as the fry of Ostrea vesiculosa. There 
is a beautiful example of Micraster cor-anguinum in the Museum 
of the Geological Society (given by Mr. Bayfield, of Norwich, 
to the late Daniel Sharpe), on which a dozen of these little 
shells, each about half an inch across, have left their lower 
valves; and above twenty are congregated on a fragment of 
Ananchytes (occupying a space of about three square inches) 
in the British Museum. 
More than a year ago my attention was particularly called to 
these shells by the Rev. Norman Glass, F'.G.S., who insisted 
on their distinctness from the young Oysters occasionally para- 
sitic on the same fossils, and the disk of which is only marked by 
the muscular impression on the left or hinder side. Mr. Glass 
also pointed out that in the smaller examples, measuring less 
than one-quarter-inch across, the border is radiated, as in the 
small attached Spondyli (‘ Dianchore’) found under the same 
circumstances. At this early stage there is no inner ridge 
dividing the striated disk from the smooth pallial region; but 
in examples like that figured (Pl. V. fig. 2), the two ridges are 
already distinguishable, and in a set of specimens the subsequent 
rapid growth of the border may be observed. ‘The plicated 
margin, wanting in the larger specimens, may also be traced to 
its extinction in the same series. The Spondyli of the Chalk 
have always lost their mner shell-layer, and exhibit a uniformly 
striated interior. It is evident that the Plicatule also have lost 
their sub-nacreous lining; these genera are very closely allied ; 
but in the recent Plicatula the (ete 4 is obsolete, the shell having 
a broad attachment, while some of the Spondyli (like Sp. spino- 
sus of the Chalk) are free. 
In the Oysters the shell is minutely vesicular between the 
lamine ; and traces of this structure may be seen, without the 
help of a glass, in all the specimens of Ostrea vesiculosa I have 
examined, from the smallest to the largest, amounting to several 
hundreds. Owing to this difference in the constitution of the 
shell, the hinge is preserved, with its ligamental area and pit; 
and the smooth interior with its adductor-scar. In some of the 
Chalk Plicatule a trace remains of the hinge-line, with its 
narrow ligamental fissure, as represented in Pl. V. fig. 3 
