PIIOLAS. 95 



Animal thick and elaviform ; the lobes of the mantle 

 united in front, except for a small space anteriorly, through 

 which the foot is protruded ; reflected dorsal ly to cover 

 the beaks, and form callosities and accessory valves. Si- 

 phonal tube long, very extensile, divided externally only 

 nearer the extremity ; orifices bordered with cirrlii, but 

 not surrounded by a cirrhigerous disk. Foot often con- 

 siderably developed, thick, short, and truncated at its 

 extremity. No tube. 



The genus Pholas, and its immediate allies, Xylopliaga 

 and Pholadidea, form a natural and graduated link between 

 such abnormal and worm-like mollusks as the Teredo^ and 

 the more ordinary and typical genera of Lamellibranchiate 

 Mollusca. The shells of some Pholades very closely re- 

 semble those of certain Petricola. The animal, whilst 

 it retains some of the peculiarities of the Teredo, espe- 

 cially in its branchial arrangements, in most respects assi- 

 milates closely to the normal lamellibranchiate types. Its 

 mantle is more highly organised than that of Teredo, and 

 the visceral mass more concentrated. The latter, includ- 

 ing the liver, the digestive organs, the large reproductive 

 glands, &c., is surmounted by a well-developed, but pecu- 

 liar foot, having a sucker-shaped truncated disk which, 

 however, is not provided with a foliaceous border like that 

 of Teredo. The stomach of Pholas is simple. The anterior 

 adductor muscle, in this genus, makes up for the deficiency 

 of ligament. The labial tentacles are largely developed. 



The species of Pholas are numerous and widel}- distri- 

 buted. The genus may be said to be cosmopolitan. Its 

 range in depth extends from low water, between tides, 

 where the majority of the species are found, to twenty-five, 

 or, })erhaps, thirty fathoms. Geologically, it dates from 

 the oolitic period. The species increase in numbers as they 



