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 peculiarity 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 therefore 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 formation.
I might perhaps cite the specimen represented in fig. 8, for the same purpose with that last referred to, as affording another example of a similar cast formed in a body distinct from the striated shell; in this specimen a beautiful groupe of globules is seen occupying a conical hollow sunk into the substance of a flint pebble, and apparently formed by the impression of the pointed end of a belemnite. It is fair however to state, that together with fragments of the striated shell, certain testaceous bodies of a tapering cylindrical
