92 



GASTROCH.ENID.E. 



at their bases, the orifice of the one, for imbibing water, fringed 

 with a circle of branching or feathery filaments consisting of fom- 

 long and four short ones ; and the same number of obtuse points 



Fig. 399. 



P. pholadiformis. 



without fringe surround the orifice of the other tube ; foot triangu- 

 lar, long, narrow, elevated ; mantle united except at the orifices for 

 the siphons and foot. Dr. Stimpson says the cirri at the orifice of 

 ihQ siphons are very variable, sometimes being entirely wanting or 

 only represented by tubercles. 



Petricola dactylus. 



Fig. 41. 



Shell elongated-ovate, clialky-white, very inequipartite, covered T\-ith radiating 

 lines and ribs ; no areola before the beaks ; teeth, two in the right, and three in 

 the left valve. 



Petricola darti/Jiis, Say (not Sowerby), Amcr. Conch, jjl. 60, fig. 2 (1834). — De Kay, Nat. 

 Hist. New York, 228, pi. 28, fig. 283. 



This shell very closely resembles the preceding, and will be best 

 described l>y a comparison with it. It has a more ovate form, the 

 basal margin being considerably arcuated : the anterior extremity is 

 higher and ol)tusely rounded ; there is no marked areola before the 

 beaks, but a deep depression under them ; the ligament is longer ; 

 the radiating lines are more numerous, the rili-like anterior ones 

 are more numerous (about sixteen), less elevated, and the lines of 

 growth merely undulate over them without being raised into vaulted 

 scales ; in the right valve are two teeth similar to those in P. p/m- 

 ladiformis, but shorter and grooved ; in the left valve, instead of tlic 

 large, cleft tooth, we have two teeth, the division between them an- 

 swering to the cleft in the other species ; the large middle tootli is 

 folded, and the posterior one is very slender. Length, one and 



