Zoological Society. 493 



Hab. Catbalonga, island of Samar, Philippines (found in coarse 

 sand and gravel at the depth of four fathoms) ; Cuming. 



Chiefly characterized by its globular form and the smooth horny 

 nature of the epidermis. 



Arca rufescens. Area testd elongafo-ovatd, valde iiKeqitivalvi, la- 

 teribus superne leviter angulatis, infra rotundatis, postico subelon- 

 gato-extenso ; albidd, riifescente tinctd, epidermide inter castas 

 partim indutd ; radiatim costatd, costis numerosis, quadragenis, 

 confertis, Icevibus ; ligamenti ared mediocri, umbonibus subtrun- 

 catis. 

 Conch. Icon. Area, pi. 8. f. 53. 



Hab. ? 



This shell is of a more solid structure than the Arca incequivalvis ; 

 the ribs are much more numerous and close-set, and independent of 

 the difference of colour, there is a truncated peculiarity in the um- 

 bones. 



Arca contraria. Arca testd oblique ovatd, solidd, turgidd, ccqui- 

 valvi, lateribus rotundatis, postico brevi, antico elongato-extenso ; 

 albidd, epidermide sericd fused crassd partim indutd ; radiatim 

 costatd, costis tricenis, Icevibus ; ligamenti ared latiusculd, declivi ; 

 umbonibus contrarie contortis. 

 Conch. Icon, Arca, pi. 8. f. 55. 



Hab. ? 



This is another conti-ary or reversed species, and a very remark- 

 able one, the shell being completely reversed, whilst the position of 

 the ligamentary area remains the same. This and the Arca reversa. 

 Gray, are the only s^jecies of the genus I have as yet seen exhibit- 

 ing this peculiarity of growth. 



Arca angicostata. Arca testd subquadrato-ovatd, subccquivalvi, 

 lateribus superne angulatis, infrh rotundatis, antico brevi ; albidd, 

 fuscescente partim tinctd, epidermide fused, inter costas squamosa, 

 indutd ; radiatim costatd, costis ad tricenas, anticis august is, sub- 

 distantibus, leviter crenulatis, posticis latioribus, expansioribus ; 

 ligamenti ared latiusculd. 



Conch. Icon. Arca, pi, 9. f. 57. 



Hab. ? 



There is a very unusual disproportion in the width of the ribs of 

 this species, the anterior ribs being exceedingly narrow, whilst the 

 posterior are wider and spread out as it were. 



Arca loricata, Arca testd subquadratd, gibbosd, lateribus superne 

 attenuato-angulatis, antico brevi, rotundato, postico angulato- 

 extenso'; albidd, corio corneo c(Eruleo-viridescente loricata, epider- 

 viide tenui, fused, inter costas squamosa, indutd ; radiatim costatd 

 costis septem vel octo et viginti, planiusculis, acutangulis ; umbo- 

 nibus subapproximatis ; ligamenti ared leviter declivi. 



Conch. Icon. Arca, pi. 9. f. 58, 



Hab. ? 



This shell is covered with a peculiarly strong homy cuticle, and 



