PHOLAS. 107 



slab of clay-slate, and placed marks iu order to see if 

 they had any rotatory motion ; but he fonnd that they 

 all invariably retained the same lateral position, and 

 that the movement was vertical onlv. When the shell 

 has been abraded or worn by rubbing against the sides 

 of its stone cell, the new layers formed in front have of 

 course their prickles, when they exist, quite perfect and 

 sharp. Specimens now and then occur which measure 

 about 6 inches in breadth. 



The synonyms are antiquated ; and two only are post- 

 Linnean, viz. P. muricatus of Da Costa, and P. Mans 

 of Pulteney. The animal is the Hypogcea verrucosa of 

 Poli. 



B. Shell oblong : dorsal shield single, posterior, and elongated. 



Bar tiea, Leach. 



2. P. can'dida*, Linne. 



P. candidus, Linn. S. N. p. 1111. P. Candida, F. & H. i. p. 117, pi. iv. 

 f. 1, 2. 



Body oblong, dirty white with a faint tinge of brown : tubes 

 more narrow, slender, and elongated than in P. dactylus ; 

 larger tube funnel-shaped, grooved inside lengthwise like 

 the barrel of a rifle, and appearing as if marked with white 

 or light-brown stripes ; its orifice is surrounded by about a 

 dozen papillae which terminate the grooves ; smaller tube 

 cylindrical, and contracted or bell-shaped at the top, with its 

 orifice either plain or surrounded by a few papillae ; sheath 

 minutely tuberculated : foot small, oval, attached by a com- 

 pressed stalk. 



Shell elongated, tumid, and thin: sculpture, 25-30 longi- 

 tudinal rows of sharp thorn-like prickles, which cover all the 

 surface except at each end, and radiate from the hinge out- 

 wards ; on the anterior side the prickles are stronger but not 

 crowded : colour chalky- white : epidermis light-brown, some- 

 what fibrous on the posterior side, and forming delicate thread- 

 like lines to connect the rows of prickles : margins rounded 



* White. 



