488 Zoological Society. 



dextralis planis, sinistralis noduloso-crenatis ; ligamenti ared me- 

 diocri. 

 Conch. Icon. Area, pi. 3. f. 16. 



Hab. Basey, island of Samar, Philippines (found in sandy mud at 

 low water) ; Cuming. 



The horny covering of this species is of an unusually transparent 

 character. 



Arca gibbosa. Area testd rotundato-ovatd, gibbosissimd, cequivalvi, 

 lateribus subangulatis, infrd rotundatis, latere antico breviore ; 

 albidd, epidermide fused indutd ; radiatim costatd, costis sex vel 

 septem et viginti, rotundatis, angustis, leviter nodulosis ; ligamenti 

 ared mediocri. 

 Conch. Icon. Arca, pi. 4. f, 20. 



Hab. ? 



This shell is almost as round as the Arca pilula, from which it 

 differs however essentially in having the umbones obliquely turned 

 to the anterior, and the ribs rounded. 



Arca maculosa. Arca testd oblique ovatd, cequivalvi, lateribus 

 superne subattenuato-angulatis, infrd rotundatis, antico brevissimo, 

 postico oblique extenso ; albidd, fuscescente tinctd, latere postico 

 maculis perpaucis fuscis ornato, epidermide tenui, corned, inter 

 costas setosd, obtectd ; radiatim costatd, costis ad sex et triginta, 

 planulatis, confertis, marginibus serratis, posticis leviter nodulosis, 

 anticis prope marginem sulco divisis ; umbonibus pallide rufescen- 

 tibus ; ligamenti ared mediocri, declivi. 

 Conch. Icon. Arca, pi. 4. f. 24. 

 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 Arca maculosa. 



Arca compacta. Arca testd subelongato-quadratd, ineequivalvi, 

 gibbosd, lateribus superne angulatis, antico breviore, postico leviter 

 angulato -extenso ; albidd, epidermide fused partim indutd; radiatim 

 costatd, costis tricenis, valvce dextralis subangustis, planulatis, 

 sinistralis latioribus, rotundatis, interstitiis cancellatis ; ligamenti 

 ared subangustd, declivi. 



Conch. Icon. Arca, pi. 5. f. 27. 



Hab. ? 



This is a short compact shell of rather solid growth. 



Arca pertusa. Arca testd suboblique quadratd, vix cequivalvi, 

 lateribus superne angulatis, infrd rotundatis ; albidd, viridescente 

 tinctd, epidermide fused sparsim indutd ; radiatim costatd, costis 

 duabus vel tribus et triginta, confertis, plano-convexis, subtilissime 

 pertusis ; ligamenti ared latiusculd, umbonibus subcentralibus. 



Conch. Icon. Arca, pi. 5. f. 28. 



Hab. Mouth of the Gambia, Africa. 



