PETRICOLA. 91 



and basal margins nearly parallel, and the extremity bluntly 

 rounded and a little gaping ; beaks elevated and inclined forwards ; 

 in front of them is a sharply ovate lunule, distinctly defined, and 

 marked only by the lines of growth ; 

 behind them is a projecting liga- ^'°- ^^' 



ment of considerable length ; sur- 

 face coarsely marked by the stages 

 of growth, and covered with ele- 

 vated, radiating lines, various in 

 size and distance ; at the posterior p. phoiadiformis. 



hinge margin they are crowded 



and very faint, while anteriorly they are large and distant ; about 

 seven or eight of them are more prominent than the rest, and the 

 lines of growth rise upon them into vaulted, toothlike scales ; hinge 

 margin very narrow ; teeth two in each valve, seeming to rise out 

 of tlie cavity of the beaks and curving upwards ; in the right valve 

 one tooth is prominent and furrowed ; the other, arising a little be- 

 fore it, and a little deeper within the shell, is quite short ; in the 

 left valve is one large, prominent tooth, so deeply divided as to re- 

 semble two, and directly behind it, diverging widely in the direction 

 of the margin, is a thin, much elevated tooth. Muscular impres- 

 sions faint, connected by a very deeply notched pallial impression ; 

 furrows within answer to rilis without. Length, one and one fourth 

 inches ; height, seven tenths of an inch ; breadth, three fifths of an 

 inch. I have seen one specimen two and one half inches long. 



Found on various parts of our coast; at Chelsea and Nahant 

 beaches it is found abundantly, imbedded in jutting fragments of a 

 marsh whicli once existed there, but which has been washed away 

 by inroads of the sea, and now only an occasional remnant lifts its 

 head above the surrounding sand. Also found in great quantities 

 boring into hard blue clay, at low-water mark, on Phillip's Beach. 

 Sable Island, rare (Willis). 



Deshayes remarks that it is a very extraordinary shell on account 

 of its exterior aspect, which would lead one to mistake it for a small 

 Pholas. To any one who has seen a Pholas, the resemldance is 

 striking ; but the want of any wide gaping, and the articulated 

 hinge, at once correct the first impression ; the teeth are so long 

 and slender that it is a rare thing to find a specimen in which some 

 of them are not fractured. 



The animal, according to the observations of the Rev. J. L, Rus- 

 sell, has two tubes or siphons extending from the longer end, united 



