CONCHOLOGY. 27 



limestone, sandstone, wood, coral, &;c. ; often discovered 

 completely imbedded in the oak planks of ships traversing 

 those seas ; as they advance in growth they enlarge the space 

 within, and leave the aperture by which they entered of its 

 primitive size. 



Shell thin, sub-transparent, finely striated, elongated oval, 

 bivalve, equivalve, inequilateral ; the valves only touching 

 in the middle of their edges ; the summits but little marked, 

 and concealed by a callosity produced by the expansion of 

 the dorsal lobes of the mantle ; near the hinge are often de- 

 veloped one or more accessory calcareous pieces ; an in- 

 curved tooth interior beneath the hinge. 



Pholas dactylus. Pholas silicula. 



P. orientalis. P. costata. 



P. Candida. P. crispata. 



p. dactyloides. P. callosa. 



p. clavata. 



P. dactylus. The prickly Pholas. PI. 3, fig. 3. 



Answers to the general description, but is beset with 

 small calcareous spiny nodules on the ribs, which run widen- 

 ing and enlarging from the summit to the margin ; colour 

 white or very light brown. 



P. striata. The striated Pholas. PI. 3, fig. 5. 



Oval, the dorsal callosity leaving the summit free, and ex- 

 tending towards the anterior and inferior extremity in such 

 a manner that each valve seems to be formed of three parts, 

 because of an oblique furrow from the summit to the mar- 

 gin ; a tooth running down in the inside from the summit ; 

 one pair of accessory pieces at the posterior extremity of 

 the shell, 



P. Candida. The white Pholas. PI. 3, fig. 2. 



Elongated, wedge-shaped ; muscular impression almost 

 medial; a kind of oblique tooth parting from the summit; 

 no accessory pieces. 



