r E T R I C L A. 



Genus PETRICOLA, Lamarcl: 

 Ti'sta ovala, plerumqne alha, mjiiiralris, epiderviidc 

 lenid imluta, interdiim radialim co^tata; latere 

 antico hrevi, rotiiiH/o, lntere postico producto, 

 frequenter attenwito. Cnrdo tilnique valca bidenlido ; 

 li/jiinientum breve, externum. Imprcsiio pal/ii 

 postice sinuata. 

 Sliell ovate, generally white, eqnivalve, covered M-ith a 

 thin epidermis, sometimes radiately ribl)e(l ; an- 

 terior side sliort, rounded, posterior side prmhu-iil, 

 frequently attenuated. Hinge with two torlh in 

 each valve; ligament short, external. I'allcal im- 

 pression posteriorly sinuous. 



Like the Venerupes, these are rnck-inliabiting shells, 

 the animals attaching themselves by means of" a byssus 

 within holes and crevices, to whicli the .shells are often 

 found to conform their shape, thus becoming irregular. 

 Generally the Petricola; are longer than the VenerupeS; 

 tapering towards the posterior, and in their sctdpturc the 

 radiating predominate over the concentric ribs or stria;. 

 Western America seems to jiresent the more favourite 

 resorts to these Mollusks, although the shores of the 

 Pacific, the Mediterranean, and Red Seas posse.ss some 

 s])ecies. India and Australia are not without them. 

 Besides seeking holes in rocks in which to reside they 

 exerc-ise the power of enlarging, if not entirely excava- 

 ting new ones, hard stones and corals being sometimes 

 pierced by them. 



Species 1. (Mus. Brit.) 



Petricola seerata. Fet. iestii. plioladiformi , ehngata, 

 aiitice hrevissimd, costis acuti's, crchris radiuU'i ; 

 postice attenuatd, striis acutis, elevatis oblique 

 radiatd, ad terminwn acuminatd. 



The serrated Petkicola. Shell pholas-shaped, elon- 

 gated, anteriorly very short, rayed with sharp, 

 close ribs ; posteriorly attenuated, obliquely rayed 

 with sharp, raised close striie, acuminated at the end. 



Desiiayes. Proc. Zool. Soc, 1853. 



Hab, New Zealand. 



Ditiering chiefly from Petricola seri-ata in the com- 

 parative fineness of the scttlpture. 



Species 2. (Mus. Brit.) 



Pktuicola gracilis. l\t. I, si, I plioladiformi, an/justd, 

 ehmgata, aiitice costis rrenululis. ilistuntibus radiatd: 

 poMice ijradatim allniinitil, rudiatim striata, ad 

 terminiim suhdeiiniiinUd. 



The si.exdki: Pi:Ti;i<ni.A. Shell pholas-hke, narrow, 

 long, anteriorly rayed with crenulated, distant 

 i-ibs ; posteriori}' gradually attenuated, radiately 

 striated, rather acuminated at the end. 



Deshayks. Proc. Znol. Soc, 181.3. 



Hal,. ? 



This shell was named by M. Deshayes from a speci- 

 men in Mr. Cuming's collection. Notwithstanding the 

 variations to which shells of this genus fere subject, the 

 extreme attenuation and finer sculpture in this case 

 may be considered sufficient to distinguish the species 

 from Petricola jiltoladifoniiis. 



S])ecies 3. (Mus. Brit.) 



Petricola coxcinxa. Pet. testa oblongd, subcylindricd, 

 antice brevi, costis crentdatis, subdi stantibus radiatd ; 

 postice productd, concentrice laminatd, inter laminas 

 radiatim striata, ad termimim truncata, 7narijiine 

 dorsali pauld decUvi. 



The xe.\t Petricola. Shell oblong, subcylindrical, 

 short in front, rayed with crenulated, rather 

 distant ribs ; posteriorly produced, concentrically 

 laminated, radiately striated between the lamina?, 

 truncated at the end, with dorsal margin a little 

 sloped. 



SowERUY. Proc. Zool. Soc, 1834, p. 46. Thesaurus 

 Conchyliorum, f. 3. 



Ilab. Monte Christi. 



The ribs at one end and the distinct and regular 



lamiufe at the other give a peculiarly pretty appearance 



to this shell. 



November, 1874. 



