Zoological Society. 487 



Hab. Island of Burias, Philippines (found in sandy mud at the 

 depth of six fathoms) ; Cuming. 



This species is so exceedingly gibbous that the shell is as wide as 

 it is high, and it is the same in all stages and varieties of growth. 



Arca anomala. Area testd quadrato-ovatd, subcompressd, incequi- 

 valvi, latere antico rotundato, postico subangulato-rotundato ; albd, 

 viridescente pallide tinctd, epidermide corned inter costas sparsim 

 indutd; radiatim costatd, costis octo et viginti, valvce dextralis 

 Icevibus, planulatis, sinistralis convexis, nodulosis ; ligamenti ared 

 angustd. 

 Conch. Icon. Arca, pi. 2. f. 9. 



Hab. ? 



It seems rather anomalous that the ribs in one valve of this shell 

 should be convex and nodulous, whilst in the other they are flat and 

 smooth ; but it is an interesting fact that the larger valve of the in- 

 equivalve species of Arca exhibits a much more elaborate style of 

 sculpture than the smaller. The Arca pilula just described shows 

 this remarkable peculiarity very distinctly, but not so prominently 

 as the species under consideration ; and it now remains to be deter- 

 mined whether this difference arises from any superiority of organi- 

 zation in the left lobe of the mantle of the animal inhabitant. 



Arca holoserica. Arca testd oblique quadratd, cequivalvi, lateribus 

 superne angulatis, antico infra rotundato, postico angulato-extenso ; 

 albidd, epidermide holosericd obtectd ; radiatim costatd, costis 

 duabus vel tribus et triginta, creberrimis, planulatis, umbones ver- 

 sus minutissime pertuso-cancellatis ; umbonibus subobliquis ; liga- 

 menti ared latiusculd. 

 Conch. Icon. Arca, pi. 2. f. 11. 



Hab. Island of Samar, Philippines (found in coarse sand and gra- 

 vel at the depth of four fathoms) ; Cuming. 



This is a very fine species, remarkable not only on account of its 

 general form and velvety epidermis, but for the minute cancellated 

 sculpture of the umbones. 



Arca gubernaculum. Arca testd subelongato-ovatd, compressius- 

 culd, incequivalvi, lateribus superne angulatis, latere antico valde 

 breviore, postico compresso-expanso ; viridescente, epidermide inter 

 costas squamosd indutd ; radiatim costatd, costis duabus vel tribus 

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



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) ; Cuming. 



This shell may be recognised by the compressly expanded charac- 

 ter of the posterior side. 



Arca cornea. Arca testd subquadratd, gibbosiusculd, incequivalvi, 

 lateribus superne angulatis, infra rotundatis, latere antico breviore ; 

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

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



