134 GUIDE TO THE FOSSIL INVEETEBRATE ANIMALS. 



Gallery [Pcihidina^ fluvioritm, in a greenish calcareous cement. The 

 Vlli. marble occurs in layers, from a few inches to a foot in thick- 

 ness, and is used for chimney-pieces, slabs, and columns. 

 It may be seen in Canterbury, Chichester, and Salisbury 

 Cathedrals, York Minster, Westminster Abbey, and the 

 Temple Church. A similar stone is found at Bethersden in 

 Kent. Other bands of limestone, often red, are full of 

 Corhicula [Gyrena] media. Pleuroccra stromhiformis ( = Fota- 

 mides carlonarius) is also noticeable. 



Table-ease 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 Dorset 

 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 Wealden, contains Corhicula [Cyrena] parva, Ifnio 

 Martini, Physa Bristovi, and the marine form Mytilm Lyelli. 

 One of the characteristic beds is the Purbeck Marble, very 

 Between like the Petworth Marble, and another is the Cinder Bed 



"^e^&^T^^^ composed of Ostrea distorta, an oyster that probably owes 

 its peculiar shape to brackish-water conditions. 



Table-cases Jurassic. Leaving the estuarine formations of inter- 

 9-14. mediate and uncertain age, we return to the marine series, 

 exhibited under a number of stratigraphical groups, which 

 will 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, S. Stutch- 

 bury, it. Damon, W. H. Hudleston, J. P. 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-eases shown ; one of them contains shells of the lamellibranchs, 

 5 & 6. Pcriia Bouchardi and Clilamys [Pecten] lamellosus ; the other 

 is almost entirely composed of shells of Ceritliium concavum. 

 In the middle of the gallery is a large block of the " Eoach- 

 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 : Trigonia gihhosa, 

 Chlamys lawtellostis, Ostrea expansa, Lucina portlandica, and 

 Protocardia dissimilis among lamellibranchs ; Natica clegans 

 and Cerithiitm p)ortlandicnm among gastropods. All these 



Table-case gj^ells may be better studied in the Table-case, as well as 



