ARCA.— Plate IV. 



Sowerby, Pro. Zool. Soc, 1833. 



Hab. Gulf of Tehuantepec, Central America (dredged 

 from the depth of twelve fathoms) ; Cuming. 

 This interesting species reminds one of the Area 

 grandis, from which it differs however not only in the 

 number of ribs, but in the colour of its hard epidermal 

 cuticle. 



Species 24. (Mus. Cuming.) 

 Arca maculosa. Arca testa oblique ovatd, aquivalvi, 

 lateribus superne subattenuato-angulatis, infra rotun- 

 datis, antico brevissimo, postico oblique extenso ; 

 albidd, fuscescente tinctd, latere postico maculis per- 

 paucis fuscis ornato, epidermide tenui, corned, inter 

 castas setosd, obtectd ; radiatim costatd, costis adsi.r 

 et triginta, planulatis, confertis, marginibus serratis, 

 posticis leviter nodulosis, anticis prope margiiicm 

 sidco dieisis; umbonibus pallide rufescentibus ; liga- 

 menti ared mediocri, declivi. 

 The spotted Ark. Shell obliquely ovate, equivalve, 

 sides somewhat attenuately angulated at the upper 

 part, rounded beneath, anterior very short, posterior 

 obliquely extended ; whitish, stained here and there 

 with faint brown, posterior side ornamented with a 

 few dark brown spots, covered with a horny epi- 

 dermis, which is bristly between the ribs ; radiately 

 ribbed, ribs about six-and-thirty in number, flat, 

 close-set, edges serrated, posterior ribs slightly no- 

 dulous, anterior divided near the margin by a 

 groove ; umbones tinged with pale red ; ligamen- 

 tary area middling, bent inwards. 

 Reeve, Pro. Zool. Soc, 1844. 

 Hab. North coast of New Holland. 



It is scarcely possible to conceive two species more 

 immediately allied than this and the Arca scapha, though 

 perfectly distinguished by the number and arrangement 

 of the ribs, which in the former are divided by a narrow 

 cut rather than a groove ; the scattered brown spots, 

 though few in number, are also peculiar to the Area 

 maculosa. 



Species 25. (Mus. Cuming.) 



Arca scapha. Arca testa oblique ovatd, cequivalvi, late- 



ribus superne angulutis, infra rotundatis, latere antico 



brevissimo, postico oblique extenso ,■ albd, epidermide 



pilosd, inter costas selosd, plus minusve indutd ; ra- 



diatim costatd, costis tribus et triginta, planulatis, 

 anticis prope marginem suleo divisis, interstitiis pos- 

 ticis cancellatis ; umbonibus pallide rufescentibus ; 

 ared ligamenti mediocri, declivi. 



The packet-boat Ark. Shell obliquely ovate, equi- 

 valve, sides angulated at the upper part, rounded 

 beneath, anterior side very short, posterior oblique- 

 ly extended ; white, more or less covered with a 

 hairy epidermis, which is bristly between the ribs ; 

 radiately ribbed, ribs three-and-thirty in number, 

 Hat, anterior ribs divided near the margin by a 

 groove, interstices between the posterior ribs can- 

 cellated ; umbones palely tinged with red ; liga- 

 mentary area middling, bent inwards. 



Chemnitz (Das paquetboot), Conch. Cab., vol. vii. p. 201. 

 pi. 55. f. 548. 



Lamarck, Anirn. sans vert., vol. vi. p. 469. 



Hub. Bay of Manila (found in sandy mud at low water) ; 



This species appears to have been confounded by 

 Limneus with the Area anliquulii. 



Species 26. (Mus. Cuming.) 



Arca emakginata. Arca testd elongato-ovatd, incequi- 

 valvi, latere antico brevissimo rotunda/o, postico 

 elongato, superne emiirginulo, infra rotundato ; al- 

 bidd, epidermide fused, inter costas setosii, obtectd; 

 radiatim costatd, cost is tricenis, planulatis, confertis. 

 anticis angustis, rugu/osis, su/co prope marginem sub- 

 obsolete divisis, posticis latioribus, hevibus ; liga- 

 menti area angustd, declivi. 



The emarginated Ark. Shell elongately oval, inequi- 

 valve, anterior side very short, rounded, posterior 

 elongated, emarginated at the upper part, rounded 

 beneath ; whitish, covered with a brown epidermis, 

 which is bristly between the ribs ; radiately ribbed, 

 ribs thirty in number, flat, close-set, anterior ribs 

 narrow, wrinkled, rather obsoletely divided near 

 the margin by a faint groove, posterior broader, 

 smooth ; ligamentary area bent inwards. 



Sowerby, Pro. Zool. Soc, 1833. 



Hab. Ileal Llejos, Atacamas, Xipixapi, Panama, Gulf of 

 California, &c (dredged from sandy mud at the 

 depth of about six or eight fathoms) ; Cuming. 

 A very interesting species, well characterised by its 



posterior emargination. 



