TESTACEA. 73 



their nature. Both inhabit the same beds of indurated clay, within the 

 fibod. There they form cavities, where they retreat with the ebb of the 

 tide, and wlience they protrude as it flows. 



Length of the shell twenty-five lines ; width fifteen, as in a line with 

 the animal. It seems an absurdity in description to speak of the diameter 

 of a tubular cavity as its length and the length as the breadth. A large 

 rib originates within, just under the umbo of the shell, and projects half 

 an inch from each valve. These ribs have a curvature corresponding 

 with the cavity of the valve. 



The animal of the Pholas, like the preceding, seems of ascidian 

 nature. The anterior is prolonged in a round form, and terminated by 

 two fringed orifices, separated by a short cleft : the posterior extremity 

 protrudes slightly, as a large round fleshy bulb or button, with a smaller 

 neck. The whole length of an animal, with a shell such as that above 

 described, is not less than four inches or more. 



Colour of the shell grey, of the animal white ; the summit of the 

 body, orifices, and fringe, variegated with brown. A thick, dark, rugous 

 integument invests the body as an epidermis, for a considerable distance 

 from the shell, wherein a very narrow cleft is lilvewise perceptible, ex- 

 posing the white flesh within. This integument belongs more particu- 

 larly to larger and older specimens. 



In captivity, the Pholas testifies little sensation or motion, remain- 

 ing constantly in the same place, and exhibiting merely extension and 

 contraction. When healthy it protrudes far from the shell ; and its 

 dimensions seem dependent on the quantity of sea water absorbed. It is 

 impatient of light — contracting even on the vicinity of a candle. 



In the natural state it must be very quiescent ; for a specimen of 

 the P. crispata occurred amidst a lump of soft sandy formation entirely 

 investing the shell, wherein only a small apcT'ture remained for protrusion 

 of the body. A fisherman also brought four specimens of the P. dacfylxs, 

 much whiter and cleaner than usual, which he affirmed he had found on 

 breaking up a piece of chalk. 



The peculiar formation of the shell is such as to fashion a cavity by 

 any motion it receives from its tenant. 



K 



