ARCA.— Plate VII. 



Hab. Coast of Africa. 



This species has a smaller number of ribs than any 

 other of the genus, and only six or seven of these are 

 at all distinctly developed. 



Species 46. (Mus. Metcalfe.) 



Arca eotundicostata. Area testd oval 



cequivalvi, lateribus siiperne angulatis, antieo brevi, 

 rotundato, postico subangulato-extenso ; alba, urn- 

 bones versus fuscescente tinctd ; radiatim costatd, 

 costis ad vicenas, subdistantibus, elevatis, rotundatis, 

 crenatis, valvce sinistralis potiils latioribus ; liga- 

 menti ared lata, paululum concavo-declivi. 



The round-ribbed Ark. Shell ovate, gibbous, equi- 

 valve, sides angulated at the upper part, anterior 

 short, rounded, posterior rather angularly extended, 

 stained with brown towards the umbones ; radiately 

 ribbed, ribs about twenty in number, rather distant, 

 rounded, crenated, those of the left valve rather the 

 wider; area of the ligament broad, a little con- 

 cavely bent inwards. 



Reeve, Pro. Zool. Soc, 1844. 



Hab. ? 



This may probably be one of the numerous species 



that have been confounded under the common title of 



Area antiquata. 



Species 47. (Mus. Cuming.) 



Arca Deshayesii. Arca testd ovato-oblongd, gibbo- 

 siusculd, cequivalvi, lateribus superne angulatis, antieo 

 brevi, rotundato, postico subelongato-extenso, leviter 

 auriculato ; albd, epidermide fused holosericd, inter 

 costas setosd, indutd ; radiatim costatd, costis ad 

 septem et viginti, rotundatis, subtilissime noduloso- 

 striatis, anterioribus sulco divisis ; ligamenti ared 

 longissimd, plano-excavatd, sulcis rhomboideis incisd; 

 umbonibus prope anticam extremitatem dispositis. 



Deshayes' Ark. Shell ovately oblong, rather gibbous, 

 equivalve, sides angulated at the upper part, ante- 

 rior short, rounded, posterior rather elongately ex- 

 tended, slightly auriculated ; white, covered with a 

 brown silken epidermis, bristly between the ribs ; 

 radiately ribbed, ribs about seven and twenty in 

 number, rounded, very finely nodulously striated, 

 the most anterior ones divided by a groove ; area of 

 the ligament very long, flatly excavated, engraved 

 with lozenge-shaped grooves ; umbones situated 

 near the anterior extremity. 



Hanley, Species of Shells, p. 155, note. 



Hab. ? 



This species may be recognised by the length of the 



cardinal area, and the manner in which the posterior 



extremity of it is compressed and inclined to become 



auriculated. 



Species 48. (Mus. Cuming ) 

 Arca clathrata. Arca testd subquadrato-ovatd, aqui- 

 valvi, lateribus leviter compressis, superne angulatis, 

 infra rotundatis ; albidd, epidermide tenui fuscescente, 

 inter costas exilissimi setosd, indutd ; radiatim cos- 

 tatd, costis leviter crenatis, interstitiis profundis, 

 lineis elevatis elathratis ; ligamenti ared subelongatd. 

 The latticed Ark. Shell somewhat squarely oval, 

 equivalve, sides slightly compressed, angulated at 

 the upper part, rounded beneath ; whitish, covered 

 with a thin, light brown epidermis, which is very 

 finely bristly between the ribs; radiately ribbed, 

 ribs slightly crenulated, interstices deep, latticed 

 with raised lines; ligamentaryarea rather elongated. 

 Reeve, Pro. Zool. Soc, 1844. 



Hab. Islands of Bureas and Ticao, Philippines (found at 

 the depth of about six fathoms) ; Cuming. 

 A small species, in which the interstices between the 

 ribs, which are very deeply engraved, are distinctly lat- 

 ticed with raised lines. 



