40 



general form and velvety epidermis, but for the minute cancellated 

 sculpture of the umbones. 



Arca gubernaculum. Jrca testd subelongato-ovatd, compressius- 

 culd, inceąuivalvi, lateribus superne angtdatis, latere antico valde 

 breviore,poslico compresso-eipanso ; viridescente, epidermide inter 

 costas sguamosd indutd ; radiatim costatd, costis duobus vel tribus 

 et triginta, planulatis, leviter noduhso-serratis ; ligamenti ared 

 angustd. 



Conch. Icon. Arca, pi. 3. f. 11. 



Hab. Basey, island of Samar, Philippines (found in coarse sand 

 and amongst shells at the depth of four fathoms) ; Guminę. 



This shell may be recognised by the compressly expanded charac- 

 ter of the posterior side. 



Arca cornea. Arca testd subguadratd, gibbosiusculd, ineecuivalvi, 

 lateribus superne angulatis, infra rotundatis, latere antico breviore ; 

 albd, corio tenui, corneo, viridescente, epidermide inter costas sgua- 

 mosd indutd; radiatim costatd, costis ad novem et viginti, valvee 

 dextraUs planis , sinigtralis noduloso-crenatis ; ligamenti ared me- 

 diocri. 



Conch. Icon. Arca, 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 

 eharacter. 



Arca gibbosa. Arca testd rotundato-ovatd, gibbosissimd, ceguivalvi, 

 lateribus subangulatis, infrci rotundatis, latere antico breviore; 

 albidd, epidermide fuscd 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 tumed 

 to the anterior, and the ribs rounded. 



Arca maculosa. Arca testd obligue ovatd, ceguivalvi, lateribus 

 superne subattenuato-angulatis, infra rotundatis, antico brevissimo, 

 postico oblique eitenso ; albidd, fuscescente tinctd, latere postico 

 maculis perpaucis fuscis ornato, epidermide tenui, corned, inter 

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

 planulatis, con/ertis, marginibus serratis, posticis leviter nodulosis, 

 anticis prope marginėm sulco divisis ; umbonibus pallide rufescen- 

 tibus ; ligamenti ared mediocri, declivi. 



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



Hab. North coast of Nevy 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, vvhich in the former are 



