class of Organic Impressions^ Ssfr, S33 



Owing to the great brittleness resulting from this texture, the 

 specimens of such shells hitherto obtained have been found in too 

 mutilated a state to authorise any definitive opinion with regard to 

 their systematic classification. We may however securely venture to 

 pronounce from the data in our possession, that there must certainly 

 have existed more than one species, and most probably more than 

 one genus of shells in which this structure prevailed ; some of the 

 fragments found may perhaps have belonged to a fossil pinna, others 

 appear rather to resemble parts of a shell of the genus ostrea. Da 

 Costa's conjecture formerly cited, which refers them to the patella 

 does not seem to be grounded on sufficient evidence. 



Had the cellular excavations occurred only in shells of this class, 

 concerning which our information Is so imperfect, it might have 

 been conjectured perhaps that they resulted from an original pecu- 

 liarity in the organization of such shells, but the occurrence of an 

 impression evidently moulded in similar cellules on the surface of a 

 cast of the echinus has proved that this cannot be the case. The 

 specimen alluded to is represented in fig. 9, the conjecture there- 

 fore originally proposed, namely that these cellules were the work 

 of animalcules preying on the shells, and on the vermes inhabiting 

 them seems to be the simplest manner of accounting for their for- 

 mation. 



I might perhaps cite the specimen represented in fig. 8, for 

 the same purpose with that last referred to, as affording another ex- 

 ample of a similar cast formed in a body distinct from the strirted 

 shell ; In this specimen a beautiful groupe of globules is seen occupy- 

 ing a conical hollow sunk into the substance of a flint pebble, and' 

 apparently formed by the impression of the pointed end of a belem- 

 nite. It is fair however to state, that together with fragments of 

 the striated shell, certain testaceous bodies of a tapering cylindrical 



