BIVALVIA. 105 
A single specimen, as above represented, is all that I have obtained or seen from the 
Upper Tertiaries of East Anglia which could be referred to this species. The artist has 
not made the figuré sufficiently elliptical, and it possesses thirty-eight fine, delicate, closely 
set ribs. Since the engraving was made, however, I have obtained some intermediate 
forms, which induce me to doubt whether the present specimen, as well as that 
figured from the Coralline Crag as varivs, may not be merely extreme modifications of 
P. opercularis. 
Prctrn pusio, Pennant. Crag Moll., vol. u, p. 33, Tab. VI, fig. 4. 
Localities. Cor. Crag passim. Red Crag, Walton and Sutton. Middle Glacial, 
Hopton. Upper Glacial, Bridlington P 
Some fragments of this species have occurred in the Middle Glacial sand, but they are 
rare. Its occurrence in the Fluvio-marine Crag seems doubtful, and I have not met with 
it from the Red Crag of Butley. There is a specimen in the British Museum presented by 
Dr. Murray with the locality of Bridlington attached to it, but it seems so unlike in colour 
and condition to the Bridlington fossils that its occurrence at that locality seems doubtful. 
Although very abundant in the Cor. Crag, I have never seen a specimen that showed 
marks of attachment by the exterior of the valve. 
Prcren opercuaris, Linn.’ Crag Moll., vol. ui, p. 35, Tab. VI, fig. 2; and Supplement, 
Tab. VIII, fig. 6. 
Localities. Cor. Crag passim. Red Crag passim. Fluvio-marine Crag, Bramerton, 
Whitlingham, Thorpe, and Bulchamp. Chillesford bed, Aldeby and Chillestord. Middle 
Glacial, Billockby and Hopton. 
The figure in Supplement represents a small shell from the Cor. Crag, which [ think 
belongs to this species; it is about half an inch in diameter. It appears to be a distor- 
tion, and resembles the figure of Ostrea arcuata, Broce. (‘ Conch. foss. Subapen.,’ tab. 
xiv, fig. 2), which is probably a distortion. ‘This species is common in some of the 
localities of the Fluvio-marine Crag, and rare at others. It occurs, though not commonly, 
in some of the localities of the Chillesford bed, but I have not met with it from either 
Horstead, Burgh, or Coltishall, or with any trace of it m the Lower Glacial sands. Ina 
fragmentary state it is very abundant in the Middle Glacial sands at Billockby, Clippesby, 
and Hopton. 
‘In the ‘ Ency. Method.,’ pl. 314, fig. 1 6, is the figure of a shell with its animal inhabitant. This 
figure which seems very fanciful, was probably intended for P. opercularis, but the animal is represented 
as having a large protruding foot on one side with two projecting siphons on the other. 
