90 GASTROCHiENIDvE. 



Hiatella minuta, Turt. Dyth. Brit. 24, pi. 2, fig. 12. —Brown, 111. Conch. Brit. 103, pi. 



47, figs. 1, IG. 

 Rhomhoidcs arctica, De Blainv. Man. pi. 80, figs. 6, 6 a, 6b, 6 r. 



Like S. rug-osa, this shell is liable to infinite variations from age 

 and place, being obliged to conform in a measure to objects around 

 it. Amid so many varieties on both sides it is hard to separate the 



species, and indeed it is not fully agreed that 

 they do not blend into one species. After much 

 observation, Forbes and llanley declare that " the 

 tangible mark of distinction between arctica and 

 N. arnica. rugosa consists in the presence, in the former, of 



var. /Mg^om. ^^^ excavated lunule in front of the beaks, which 



are moreover acute, leaning forward, and, when viewed in front, suf- 

 ficiently prominent. The anterior extremity is more or less cunei- 

 form, and is always attenuated ; whereas in riig-osa that portion is 

 usually rounded and frequently Ijroad ; in that shell, likewise, the 

 downward inclination of the front dorsal margin is almost invariably 

 arcuated or convex, whilst in the i)rcsent species it is oblique, and 

 for a considerable distance incurved, only becoming convex near its 

 ventral termination, which consequently is its most projecting part ; 

 the chief prominence in rug-oaa being, on the contrary, usually sit- 

 uated nearer the dorsal side." 



Ociius PETRICOLA, Lam. (1801.) 



Shell elongated, inequipartite, rounded before, narrowed back- 

 wards ; hinge almost toothless ; ligament exterior. 



Petricola pholadiformis. 



Shell ovate-cylindrical, chalky white, very inequipartite, acutely rounded be- 

 fore, 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. 



Petricola pholadiformis. Lam. An. sans Vert. 2(1 ed. vi. 1,59. — Desiiayks, Encyc. Meth. 

 Vers, iii. 747. — Sowerby, Genera {Petricola), figs. 1, 2 ; Thes. Conch, pi. 166, fig. 1. 



— Say, Anicr. Conch, pi. 60, fig. 1 (1834). — Conrad, Amer. Mar. Conch, pi. 7. 



— CiiENu, ISLan. do Conch, ii. 100, figs. 446, 447. — Reeve, Elcm. Conch, ii. 13.5, 

 f5^r. 212. — De Kay, Nat. Hist. New York, 228, pi. 28, fig. 282. 



Petricolii fornirata. Say, Journ. Ac. Nat. Sc. ii. 319 (1822). — Russell, Essex Co. See. 

 Jonrn. i. 55. 



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

 without ; cquivalve, very inequipartite, the anterior part very short 

 and acutely rounded ; posteriorly very little narrowed, the hinge 



