90 gastroch^nidj:. 



IliateJla mbmta, TuRT. Dytli. Brit. 24, pi. 2, %. 12. — Browx, 111. Conch. Brit. 103, pi. 



47, figs. 1, 1(5. 

 lihoinboides arclicu, De Blainv. Man. pi. 80, figs. 0, Ga, (jh, 6c. 



Like B. rufi^'osa, this shell is liahlc to iniiuitc variations from age 

 and })lace, l)oing obliged to conform in a measure to ol)jects around 

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

 species, and indeed it is not fullj agreed that 

 they do not l)lend into one species. After much 

 observation, Forbes and llanley declare that "■ the 

 tangible mark of distinction between arctica and 

 N. antira. rug'usd coiislsts 111 tlic pi'csence, in the former, of 



^''^'■'"S"^"- .-jn 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 ; Avheicas in rugosa that ])ortion 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 conseciuently is its most ])rojecting part ; 

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

 uated nearer the dorsal side." 



Oenus PETRICOLA, Lam. (ISOl.) 



Shell elongated, inequipartite, rounded licfore, narrowed back- 

 wards ; hinge almost toothless ; ligament exterior. 



Petricola pholadiformis. 



Shell ovato-cvlinclrical, 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. 2d cd. vi. 1.59. — Deshayes, Encyc. Mc'th. 

 Vers, iii. 747. — Sowerbv, Genera {Petricola}, fi,i;s. 1,2; Thes. Coneh. pi. 166, fig. 1. 



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



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

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



Petricola fornicata, Say, Joiirn. Ac. Nat. Sc. ii. 319 (1822). — Russell, Essex Co. Soe. 

 Journ. i. 55. 



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

 without; equivalve, very ine(iuipartite, the anterior i)art very short 

 and acutely rounded ; posteriorly very little narrowed, the hinge 



