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done, the tubular sheath which enclosed the animal and its real shell, as 

 the shell itself. But making further researches, he perceived a consi- 

 derable agreement between the valves of Modiola and those of Fistulana; 

 and learned that some fistulanae passed out of their proper tube, and lived 

 in the tubular cavities formed in stone and other solid bodies, those cavi- 

 ties 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 acces- 

 sory piece, forming the cavity in which the animal was to reside. 



On this principle, he thinks that the tube of the Fistulana, Teredo, and 

 Periicillus, 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 therefore should be 

 used only in the distinction of their genera*. 



Agreeable to this arrangement, we will proceed to the consideration of 

 cquivalved bivalves with accessory pieces. 



CXXVII. Pholas. A transverse gaping shell, composed of two large 

 principal valves, with many small accessory pieces placed on the liga- 

 ment or at the hinge. 



Among the Essex fossils are frequently found fragments of P. crispata; 

 but, from the extreme brittleness of the shell, the fragments are in gene- 

 ral but small. I have, however, on a late trip to Walton, been so suc- 

 cessful 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 which it is marked, 



* 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 circumstance 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 coosi- 

 as a tubular than a bivalve shell. 



