90 GASTROCH.ENID.E. 



Iliatella minuta, Tuet. Dyth. Brit. 24, pi. 2, fig. 12.— Brown, III. Conch. Brit. 103, pi. 



47, figs. 1, 16. 

 Ehomhoides arctica, De Blainv. Man. pi. 80, figs. 6, 6 a, 6 Z*, 6 c. 



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 lioth 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 

 oljservation, Forbes and Hanley declare that " the 

 tangible mark of distinction between arctica and 

 '^sTZ^um. rug-osa consists in the presence, in the former, of 



var. rugnsa. ^^^ cxcavatcd luiiulc ill froiit 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; wheieas in rugosa that portion is 

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

 downward inclination of the front dorsal margin is almost invariably 

 arcuated or convex, whilst in the present 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 rugosa being, on the contrary, usually sit- 

 uated nearer the dorsal side." 



Oemis PETRICOLA, Lam. (1801.) 



Shell elongated, inequipartite, rounded before, narrowed Imck- 

 wards ; hinge almost toothless ; ligament exterior. 



Petricola pholadiformis. 



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

 fore, covered witli 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. 2d ed. vi. 1.59. — Deshayes, Encyc. Mcth. 

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



— Say, Amcr. Conch, pi. 60, fig. 1 (18.34). — Conrad, Amer. Mar. Conch, pi. 7. 



— Chenu, Man. de Conch, ii. 100, figs. 446, 447. — Reeve, Elcm. Conch, ii. 13.5, 

 ficr. 212.— Dk Kay, Nat. Hist. New York, 228, jil. 28, fig. 282. 



Petricola for 11 irata, Say, Jonrn. Ac. Nat. Sc. ii. 319 (1822). — Russell, Essex Co. Soc. 

 Jonni. i. 55. 



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

 without ; equivalve, very inequiijartite, the anterior part very short 

 and acutely rounded ; posteriorly very little narrowed, the hinge 



