A RCA. —Plate VI. 



Reeve, Pro. Zool. Soc, 1844. 

 Hab. Zanzibar. 



The great width of the ligamentary area, separating 

 the umbones asunder to a considerable extent, imparts 

 a wedge-like form to this shell, by which it may be 

 easily recognised. 



Species 38. (Mus. Brit.) 



Arca secticostata. Arca testa elongato-ovatd, gibbo- 

 sissimd, tenuiculd, lateribus superne ungulatis, infra 

 suboblique rotundatis ; albidd, fuscescente partim 

 tinctd; radiatim costatd, costis numerosis, angustis, 

 ad quadragenas, quamplurimis sulcosubpro fundo di- 

 visis ; Ugamenti ared elongatd, latiusculd ; umbonibus 

 tumidis. 



The cut-ribbed Ark. Shell elongately ovate, very 

 gibbous, rather thin, sides angulated at the upper 

 part, rather obliquely rounded beneath; whitish, 

 partially stained with light rusty brown ; radiately 

 ribbed, ribs numerous, narrow, about forty in num- 

 ber, most of which are cut or divided by a rather 

 deep groove ; area of the ligament elongated, rather 

 wide ; umbones full. 



Reeve, Pro. Zool. Soc, 1S44. 



Hab. ? 



This shell is probably one of those that have been 



confounded with the Arca antiquata (Plate IX ) ; it 



differs in having a much greater number of ribs, and a 



larger proportion of them more distinctly grooved. 



Species 39. (Mus. Cuming.) 

 Arca ferruginea. Arca testa ovato-oblongd, aquivalvi, 

 lateribus rotundatis ; albidd, epidermide ferrugineo- 

 fuscd indutd ; radiatim costatd, costis ad septem et 

 viginti, angustis, compressis, subtilissime nodulosis ; 

 Ugamenti ared subangustd ; umbonibus oblique in- 

 curvis. 

 The rusty Ark. Shell ovately oblong, equivalve, sides 

 rounded ; whitish, covered with a rusty brown epi- 

 dermis ; radiately ribbed, ribs about twenty-seven 

 , in number, narrow, compressed, very finely nodu- 

 lous ; area of the ligament rather narrow; umbones 

 obliquely incurved. 

 Reeve, Pro. Zool. Soc, 1S44. 



Hab. ? 



The ribs of this species are unusually narrow, and 



being finely nodulous, present a kind of beaded appear - 



Species 40. (Mus. Cuming.) 



Arca radiata. Arca testa ovato-oblongd, aquivalvi, la- 

 teribus rotundatis ; albidd, fuscescente ad umbones 

 tinctd, epidermide sparsim indutd ; radiatim costatd, 

 costis ad tricenas, angustissimis, obsolete nodulosis, 

 inte?-stitiis clathratis ; Ugamenti ared mediocri. 



The rayed Ark. Shell ovately oblong, equivalve, sides 

 rounded ; whitish, stained with light brown about 

 the umbones, sparingly covered with epidermis ; 

 radiately ribbed, ribs about thirty in number, very 

 narrow, obsoletely noduled, interstices between the 

 ribs latticed ; area of the ligament middling. 



Reeve, Pro. Zool. Soc, 1844. 



Hab. ? 



The ribs are much less prominently developed than 



in the preceding species ; they are also narrower, more 



numerous, and less strongly noduled. 



Species 41. (Mus. Hankey.) 



Akca obliqua. Arca testd ovatd, vald'e obliqud, eequi- 

 va/ri, lateribus superne angulatis, infra rotundatis. 

 antico brevissimo, postico oblique expanso ; albd, epi- 

 dermide fuscescente scried, inter castas setosd, indutd; 

 radiatim costatd, costis quinque vel sex et triginta, 

 planulutis, confertis, interstitiis subtilissime striatis, 

 anlicis posticisque leviter crenatis, anticis sulco di- 

 visis ; Ugamenti ared subangustii, declivi • umbonibus 

 approximatis . 



The oblique Ark. Shell ovate, very oblique, equivalve, 

 sides angulated at the upper part, rounded beneath, 

 anterior side very short, posterior obliquely ex- 

 panded ; white, covered with a light brown, silken 

 epidermis, bristly between the ribs ; radiately 

 ribbed, ribs five or six and thirty in number, fiat, 

 close-set, interstices between them very finely 

 striated, the most anterior and posterior ribs lightly 

 crenated, the anterior divided by a groove; area 

 of the ligament rather narrow, bent inwards ; um- 

 bones approximated. 



Reeve, Pro. Zool. Soc, 1844. 



Hab. West coast of Africa. 



This is a very interesting species, and altogether one 



of very distinct character. 



