ARCA.— Plate II. 



Species 11. (Mus. Cuming.) 

 Arca holoserica. Area teste! oblique quadrate!, aqui- 

 valvi, lateribus superne angulatis, antico infra ro- 

 tundato, posiico angulato-extenso ; albidd, epidermide 

 holosericd obtectd ; radiatim costatd, costis duabus vel 

 tribus et triginta, creberrimis , planulatis, umbones 

 versus minutissime pertuso-cancellatis ; umbonibus 

 subobliquis ; ligament! area latiusculd. 



The velvety Ark. Shell obliquely quadrate, equi- 

 valve, side angulated at the upper part, anterior 

 rounded below, posterior angularly stretched out ; 

 whitish, covered with a peculiarly soft, short, vel- 

 vety epidermis ; radiately ribbed, ribs two or three 

 and thirty in number, very close-set, flat, very mi- 

 nutely cancellated in a pricked manner near the 

 umbones ; umbones a little oblique ; area of the 

 ligament rather wide. 



Reeve, Pro. Zool. Soc, 1844. 



Hub. Island of Samar, Philippines (found in coarse sand 



and gravel at the depth of four fathoms) ; Cuming. 



This is a very fine species, remarkable not only on 



account of its general form and velvety epidermis, but 



for the minute cancellated sculpture of the umbones. 



Species 12. (Mus. Metcalfe.) 

 Arca rhombea. Arca testa quadrato-cordatd, eequivalvi, 

 lateribus superne angulato-coarctatis, infra rotunda- 

 tis ; albidd, pa/lid'e rubiginoso tinctd; epidermide cor- 

 ned, fused, sapissime exutd ;. radiatim costatd, costis 

 subangustis, rotundatis, an/icis nodulosis ; umbonibus 

 prominentibus, valde erectis, subremotis ; ligament! 

 ared latiusculd. 

 The rhomboid Ark. Shell somewhat squarely heart- 

 shaped, equivalve, sides angularly contracted at the 

 upper part, rounded below ; whitish, stained with 



pale rusty red; epidermis brown, horny, generally 

 stripped off ; radiately ribbed, ribs rather narrow, 

 rounded, anterior ribs nodulous ; umbones promi- 

 nent, very erect, rather remote from each other ; 

 area of the ligament rather wide. 

 Born, Mus., p. 90. 



Chemnitz, Conch. Cab., vol. vii. pi. 56. f. 553. 

 Hab. Chinese Seas, Ceylon. 



This species is very inadequately described by La- 

 marck ; the anterior ribs are nodulous, and care must 

 be taken not to confound it with the Arca granosa. 



Species 13. (Mus. Metcalfe.) 

 Arca transversa. Area testa elongato-ovatd , inriquivalvi, 

 lateribus superne angulatis, infra rotundatis, postico 

 subangulato-ejctenso ; albidd, epidermide corned prope 

 marginem indutd; radiatim costatd, costis tricenis, 

 planulatis, viilrtc de.rlralis anlicis, siuistralis omnibus, 

 leviter nodulosis ; umbonibus depresso-approximatis ; 

 ligament! and august a. 

 The transverse Ark. Shell elongately ovate, inequi- 

 valve, sides angulated at the upper part, rounded 

 below, posterior side somewhat angularly extended ; 

 whitish, covered near the margin with a horny epi- 

 dermis; radiately ribbed, ribs thirty in number, flat; 

 anterior ribs of the right valve, all the ribs of the 

 left valve, slightly nodulous ; umbones depressly ap- 

 proximated ; area of the ligament narrow. 

 Say, Journ. Acad. Nat. Sci. Philadelphia. 

 Hab. United States; Say. 



The length of a bivalve shell being unquestionably 

 from the anterior to the posterior extremity, agreeable to 

 the natural position of its animal inhabitant, the title of 

 ' transversa' is inappropriate as applied to this species. 



