LITHOPHAGID.E. MOLLUSCA. PETRICOLA. 63 



respond well with the description of S. pholadis, LAM., the My a 

 byssifera of FABR. These are found in places where their regular 

 growth is unobstructed. Other specimens, and especially adult ones, 

 seem not to vary from S. rugosa. Nothing could apply better to our 

 shell, than Turton's description of My'tilus rugbsus, in his "Con- 

 chological Dictionary." But our shell is less likely to belong to this 

 than to S. pholadis, inasmuch as, besides the presence of a byssus, our 

 shell is not a borer like S. rugosa ; indeed, there are no rocks on our 

 coast of a calcareous nature. 



Again, there are small specimens in which the two lines or ridges 

 along the posterior slope, armed with spines, are very conspicuous, 

 corresponding to the S. rhomboides of Deshayes, the My'tilus prczci- 

 sus of Montagu, and doubtless the Hiatella drctica of Lamarck, 

 (Solen miniitus, LIN.) 



Now all these varieties are found living promiscuously together ; 

 and, as their shape is known to be greatly modified by the circum- 

 stances under which they are developed, the rational conclusion is, 

 that they all pertain to the same species ; and the probability is, that 

 they are identical with the European shell ; but under what name to 

 place them, and whether under one or more, it is now impossible to say. 



GENUS PETRICOLA, LAM. 



Shell transverse, inequilateral, rounded before, narrowed posteri- 

 orly ; hinge almost toothless ; ligament exterior. 



PETRICOLA PHOLADIFORMIS. 



Shell ovate-cylindrical, chalky-white, very inequilateral, acutely 

 rounded before, covered with elevated radiating lines and ribs ; an 

 ovate areola before the beaks ; teeth two in each valve, one in the 

 left valve deeply cleft. 



State Coll., No. 244. Soc. Cab., No. 1746. , 



Petricola pholadiformis, LAM. ; An. sans Vert.,v\. 159. DESHAYES ; Encyc.Meth., 

 Vers, iii. 747. SOWERBY; Genera, Petricola, f. 1,2. SAY; Amer. Conch., pi. 

 60, f. 1. CONRAD ; Amer. Mar. Conch., pi. 7. 



Petrieola fbrnicata, SAY ; Journ. Acad. Nat. Sc. t ii. 319. RUSSELL ; Essex Co. 

 Soc. Journ., i. 55. 



Shell much elongated, ovate-cylindrical, chalky-white within 

 and without ; equivalve, very inequilateral, the anterior portion 



