493 Zoological Society. 



Arca rotundicostata. Area testa cvatd, gibbosd, (Equivalvi, la- 

 terihis supernt angulatis, antieo brevi, rotundato, postico suban- 

 gulato-extenso ^ albd, umbones versus fusccseente tinctd ; radiatitn 

 costatd, costis ad vicenas, subdistantibus, elevatis, rotundatis, cre- 

 natis, valvce sinistralis potiUs latioribus ; ligamenti ared lata, pau- 

 lulUm concavo-declivi. 



Conch. Icon. Arca, pi. 7. f. 46. 



Hub. ? 



A species in which the ribs are peculiarly rounded. 



Arca clathrata. Arca testd subquadrato-ovatd, eequivalvi, late- 

 ribus leviter compressis, superne angulatis, infrd rotundatis; 

 albidd, epidermide tenuifuscescente, inter castas exilissime setosd, 

 indutd ; radiatim costatd, costis leviter crenatis, interstitiis pro- 

 fundis, lineis elevatis clathratis ; ligamenti ared subelongatd. 



Conch. Icon. Arca, pi. 7. f. 48. 



Hub. Islands of Burias 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 latticed with raised lines. 



Arca ovata. Arca testd subelongato-ovatd, tenui, valde incequivalvi, 

 lateribus superne angulato-rotundatis, postico breviore, subtiis an- 

 gulato-extenso, antieo attenuato-rotundato ; albidd, rufescente-fusco 

 tinctd, epidermide tenuissimd ; radiatim costatd, costis ad tricenas, 

 anticis nodulosis, posticis minus distinctioribus, planulatis, laevibus, 

 costis valvcE sinistralis valde latioribus ; ligamenti ared parvd, 

 latiusculd. 



Conch. Icon. Arca, pi. 8. f. 49. 



Hab. St. Elena, South America (found in sandy mud at the depth 

 of from six to eight fathoms) ; Cuming. 



This shell partakes of the characters of the Arc(E Brasiliana and 

 incongrua ; it is however more elongated than either, and the ribs are 

 narrower and more in number. 



Arca crenata. Arca testd subelongato-quadratd, subcompressd, 

 soUdiusculd, vix eequivalvi, lateribus angulato-rotundatis ; albidd, 

 epidermide tenui fused indutd; radiatim costatd, costis septem vel 

 octo et triginta, valvarum ambarum omnibus noduloso-crenatis ; 

 ligamenti ared subangustd, declivi, umbonibus depresso-approxi- 

 matis. 



Conch. Icon. Arca, pi. 8. f. 51. 



Hab. } 



This species may be distinguished by its multiplicity of ribs, 

 coupled with the manner in which the whole of them in both valves 

 are crenated. 



Arca globosa. Arca testd globosd, tumidd, subeequivalvi, lateribus 

 superne subangulatis , antieo breviore ; albidd, epidermide corned 

 fused indutd ; radiatim costatd, costis duabus vel tribus et triginta, 

 Icevibus, planulatis ; ligamenti aredmediocri, umbonibus latiusculis, 

 subapproximatis. 



Conch. Icon. Arca, pi. 8. f. 52. 



