1850.] MORRIS AND LYCETT ON PACHYRISMA. 399 



with in the midst of beds of the upper sandstone, we have proof of 

 the occasional operation of currents transporting matter in a direction 

 opposite to the general prevailing current during this period. 



The almost entire absence of the beds of red and white clay which 

 commonly occur in this formation in other localities, may probably 

 have resulted from the currents having been too powerful for the de- 

 position of an argillaceous sediment. 



May 8, 1850. 

 Lord Alfred Churchill was elected a Fellow. 

 The following communications were read : — 



1 . On Pachyrisma, a Fossil genus o/Lamellibranchiate Con- 

 CHiFERA. By John Morris, F.G.S. and John Lycett, Esq. 



Among the fossil shells of the oolite undescribed or imperfectly 

 known, is one, only recently discovered, which is remarkable as pre- 

 senting characters tending to approximate certain known genera here- 

 tofore widely separated. The locality of this genus is one pecu- 

 liarly rich in the remains of Testacea. These will be illustrated in a 

 forthcoming memoir to be published by the Palseontographical Society. 

 The locality belongs to the Minchinhampton district of the great 

 oolite formation, the geological position of the shell being as follows. 

 About 70 feet above the Fuller's earth is the base-line of a series of 

 limestone beds, which have an aggregate thickness exceeding 20 feet ; 

 the lowest bed is a calcareo-siliceous rock, of a cream colour, exhaling 

 an argillaceous odour when breathed upon, and is peculiarly compact 

 and homogeneous in its structure. Its two di\isions have a united 

 thickness of 5 feet. The beds above this have a browner aspect, are 

 usually less compact and homogeneous, and are for the most part 

 destitute of organic remains, although locally these are met with in 

 some abundance. 



Pachyrisma occurs in a vertical thickness of only half a yard ; it 

 occupies the upper 9 inches of the lowest or white bed, and the en- 

 tire thickness of the superimposed browner bed ; the shells are clus- 

 tered together in great numbers, the valves being both separated and 

 in apposition ; and it is worthy of notice that they constituted almost 

 the sole testaceous animals within the narrow limits of their habitat ; 

 no other bivalve is found here, and the univalves consist only of a 

 few casts of two species of our new genus Purpuroidea and two of 

 Natica. 



The locality in question having been under our notice for some 

 years, we are enabled to state, with a near approach to certainty, that 

 the area wherein Pachyrisma is clustered occupies but a few square 

 yards, and that beyond that space it is rare. On stepping forty yards 

 to the northward, where other excavations are in progress, we find the 

 same bed of white limestone, the upper part of which likewise contains 



