134 GUIDE TO THE FOSSIL INVEETEBIIATE ANIMALS. 
YPaliL(lina\ Jluviorum, in a greenish calcareous cement. Tlie 
marble occur.s in layers, from a few inches to a foot in tliick- 
ness, and is used for chimney-i)ieces, shib.s, and columns. 
It may be seen in Canterbury, Chichester, and Salisbury 
Cathedrals, York IMinster, Y'estminster Abbey, and the 
lemple Church. A similar stone is IVmnd at Bethersden in 
Kent. Oilier bands of limestone, often red, are full of 
Corhicnla \Ci/rcna\ media. Pleuroccra stromlrijormis { = Poia- 
mides carhunarins) is also noticeable. 
Further west in the same great estuary were deposited 
the Purbeck Beds, the uppermost of which are by some 
geologists regarded as Cretaceous, while the lower are 
Jurassic. They extend from the Isle of Purbeck in iJorset 
to Brill in Buckinghamshire, and are found also at Brightling 
and Pounceford in Sussex. A small set of shells from them 
is shown, and in addition to some .species already observed 
in the AN'ealden, contains Corhicula \Cyreiut\ farva, Ifnio 
Martini, Physa Bristovi, and the marine form Mytilus Lijclli. 
One of the characteristic beds is the Purbeck Marble, very 
Between like the Petworth Marble, and another is the Cinder Bed 
6 composed of Ostrea distorta, an oyster that probably owes 
its peculiar shape to brackish-waWr conditions. 
Table-cases Jurassic. Leaving the estuarine formations of inter- 
mediate and uncertain age, we return to the marine series, 
exhibited under a number of stratigraphical groups, which 
Avill be taken in order, beginning with the highest. Many 
of these British Jurassic specimens in the Museum are of 
historical importance, having been described and figured 
by J. Sowerby, J. Phillips, Morris & Lycett, 8. 8tutch- 
bury, li. Damon, II. Hudleston, J. F. Blake, and others. 
The Portland Oolite, from which is derived the name of 
the Portlandian Age, is worked for building stone in Dorset 
Between and Wiltshire. Two slabs of the hard Portland stone are 
Wall-cases shown ; one of them contains shells of the lamellibranchs, 
5 & 0. Perna Bouchardi and CJdamys \^Pccteii\ lamclloms ; the other 
is almost entirely composed of shells of Ccrithium concavum. 
In the middle of the gallery is a large block of the “ Boach- 
bed,” which is full of hollows, whence the shells have been 
dissolved by percolating water, leaving behind impressions 
and internal casts of the following species: Triyonia ffibbosa, 
Chlamys lumclloms, Ostrea exjiansa, Lneina loortlamhca, and 
Protoeardia dissimilis among lamellibranchs ; Natiea cleyans 
and Ccrithium portlandieum among gastropods. All these 
Table-case gjigUs may be better studied in the Table-case, as well as 
Gallery 
VIII. 
Table-case 
9. 
