BIVALVES. 



PLATE LVL 



Genus. PHOLAS. 



Character. Shell ovate, diminishing, unequally gaping at the ends, umbonated, 

 having four operculatory valves fixed near the hinge ; the outside tuberculous. 



Species. 



No. \. and 2. Pholas marmoratus. Shell of a pale brown colour, richly orna- 

 mented with waved ridges, diversely striated; the hinge joined by four 

 oblong and pointed valves. This singular and elegant shell is found 

 inclosed in rocks of stone and sometimes of marble on the coasts of France, 

 near Quiberon, also in Italy not unfrequently in a fossil state. It has been 

 a perplexing enquiry to Naturalists at all times, and hitherto an unsuccess- 

 ful one, by what principle shell-fish are enabled to penetrate the solid sub- 

 stances of rock, wood, marble, and flint : after much examination it appears 

 most likely to be effected by some acid fluid which the animal has the 

 power of using as a solvent for the above substances. As it is not supposed 

 that these substances constitute the principal part of their food, we are 

 quite at a loss to conjecture why they seek a secluded situation ; it may, 

 perhaps, be for the purpose of affording security to their offspring during a 

 certain period of their growth. 



No. 3. The side view of the above shell, shewing the opening and hinge. 



Genus. MYA, or SHIELD-SHELL. 



Character. Shell sometimes deeply striated, and umbonated like a shield, one 

 extremity or end gaping, the other closed, the first tooth solid, thickened, and not 

 inserted into the opposite shell. 

 No. 1. Mya particolor. Shell barred with red and purple veins, in round and 



spreading shape. Supposed to be a native of the East Indies. From the 



original in the British Museum. Very rare. 

 No. 2. Delineation of the hinge of the same shell. 



REMARKS. 



The genera Pholas and Mya bear a distant resemblance to each other, in many respects : 

 the former may be always sufficiently distinguished by the singular form of its hinge, which 

 differs from all others at present known. It is sometimes found, like the Teredo, inclosed in 

 logs of wood, or old trees, which have fallen into the sea. The Mya has a very considerable 

 resemblance to the ancient shield said by Plutarch to have been borne by the Amazons. It 

 is also remarkable by having a gaping or opening at one end, and being closed in front. 

 Several species of this genus are found in a fossil state in Berkshire and Hampshire, but 

 different in form from all the recent ones hitherto discovered. 



