24 MULTIVALVES. PHOLAS, 



PHOLAS. — Stone-piercer. 



Animal — aiiAscidia: Shell bivalve, divaricate, with seve- 

 ral smaller, differently shaped accessory valves at the 

 hinge; hinges recurved, united by a cartilage ; in the in- 

 side, beneath the hinge, is an incurved tooth. 



It appears that hitherto only twelve species of this ge- 

 nus have been enumerated, and some of those are so alike, 

 that in many instances they may be considered as mere 

 varieties, rather than different species; however, they all 

 possess sufficient determinate characters, to prevent any 

 admixture with the genera of Bivalves. 



The form of the Pholas is in most species ovate or ob- 

 long, which is constituted by two large valves, being si- 

 tuated opposite to each other, and to which is attached, 

 in the vicinity of the beaks, a number of smaller ones, 

 serving as substitutes for a hinge, which, in bivalves, or 

 shells of two opposite parts only, often determines their 

 generic character. 



Another character of the Pholas is, that the valves, 

 (i. e. the two large ones), never shut close, they are inva- 

 riably open at one end, and, in most instances, at both. 



In the interior of the shell, in each valve, nearly under 

 the beak, is an incurved tooth, sometimes of considerable 

 length, and which may certainly be considered as a pe- 

 culiarity of the genus. 



The exterior of the Pholas is generally destitute of co- 

 lor; sometimes it partakes of a brownish cast, but the 



