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and its real shell, as the shell itself. But making further researches, 
he perceived a considerable agreement between the valves of Modiola 
and those of Fistulana ; and learned that some fistulanse passed out of 
their proper tube, and lived in the tubular cavities formed in stone 
and other solid bodies, those cavities supplying the place of their own. 
Hence he concluded, that the two equal valves which adheres to the 
animal, formed the true shell of Fistulana ; and that the shelly tube 
should only be regarded as an accessory piece, forming the cavity in 
which the animal was to reside. 
On this principle, he thinks that the tube of the Fistulana, Teredo, 
and Penicillus, as well as the accessory portions of shell of the Pholas, 
should be considered as objects independent of the general character 
which should be employed in the classing of these animals, and there- 
fore should be used only in the distinction of their genera*. 
Agreeable to this arrangement, we will proceed to the consideration 
of equivalved bivalves with accessory pieces. 
CXXVII. Pholas. A transverse gaping shell, composed of two 
large principal valves, with many small accessory pieces placed on 
the ligament or at the hinge. 
Amongthe Essex fossils are frequently found fragments of P. crispata; 
but, from the extreme brittleness of the shell, the fragments are in 
general but small. I have, however, on a late trip to Walton, been 
so successful as to obtain a fragment or two, possessing the recurved 
tooth, which, together with the crinkled surface, leaves little room for 
doubting it to be of the species P. crispatus. I also obtained a small 
perfect shell, which from its form, and the fineness of the striae with 
* Agreeable to this last decision of Lamarck, Penicillus should be placed among the bivalves 
instead of among the tubular shells, as it will be found in the eighth Letter. But one circum- 
stance seems to render the propriety of its removal to this place doubtful — the valves in the 
genus Penicillus are incrusted in it, and form a part of it ; consequently, as they cannot perform 
the office of valves, the dwelling of the animal may be rather considered as a tubular than a 
bivalve shell. 
