ARCA.— Plate VIII. 



Hub. Catbalonga, Island of Samar, Philippines (found 

 in coarse sand and gravel at the depth of four fa- 

 thoms) ; Cuming. 

 Chiefly characterized by its globular form, and the 



smooth horny nature of the epidermis. 



Species 53. (Mus. Cuming.) 

 Arca rufescens. Arca testd elongato-ovatd, valde in- 

 (cquivalvi, lateribus superne leviter angulatis, infrd 

 rotundatis, postico subelongato-extenso ; albidd, ru- 

 fescente tinetd, epidermide inter costas partim indutd ; 

 radiatim costatd, costis numerosis, quadragenis, con- 

 fertis, Icevibus ; ligamenti area mediocri, umbonibus 

 subtruncatis. 

 The red-tinged Ark. Shell elongately ovate, very 

 inequivalve, sides slightly angulated at the upper 

 part, rounded beneath, posterior side rather elon- 

 gately extended ; whitish, tinged with red, par- 

 tially covered between the ribs with an epidermis ; 

 radiately ribbed, ribs numerous, forty in number, 

 close-set ; area of the ligament middling, umbones 

 somewhat truncated. 

 Reeve, Pro. Zool. Soc, 1844. 



Hab. ? 



This shell is of a more solid structure than the Arca 

 inaquivalvis ; the ribs are much more numerous and 

 close-set, and independent of the difference of colour, 

 there is a truncated peculiarity in the umbones. 



Species 54. (Mus. Saul.) 



Arca in^quivalvis. Arca testd elongato-ovatd, valde 

 ituequivaivi, lateribus superne angulatis, infrd rotun- 

 datis, postico subc.rtcnso ; albidd, virideseente tinetd, 

 epidermide fused, inter costas setosd, partim indutd ; 

 radiatim costatd, costis duabus vel tribus et triginta, 

 lavibus, planulatis ; ligamenti ared subangustd, um- 

 bonibus paululilm approximatis. 



The inequivalve Ark. Shell elongately ovate, very 



inequivalve, sides angulated at the upper part, 

 rounded beneath, posterior side somewhat ex- 

 tended ; whitish, stained with light green, partially 

 covered with a brown epidermis, bristly between 

 the ribs ; radiately ribbed, ribs two or three and 

 thirty in number, smooth, flat ; area of the liga- 

 ment rather narrow, umbones a little approxi- 

 mated. 



Bruguiere, Enc. Meth. vers, vol. i. part i. p. 106. 



Hab. ? 



The ribs of this species are sculptured, as it were, with 



a remarkable degree of sharpness and regularity ; the 



umbones are full, and rather inclined to approximate 



over the ligamentary area. 



Species 55. (Mus. Cuming.) 



Arca contraria. Arca testd oblique ovatd, solidd, 

 turgidd, aquivalvi, lateribus rotundatis, postico brevi, 

 antico elongato-extenso ; albidd, epidermide sericd 

 fused crassd partim indutd ; radiatim costatd, costis 

 tricenis, Uevibus ; ligamenti ared latiusculd, declivi ; 

 umbonibus contrari'e contortis. 

 The contrary Ark. Shell obliquely ovate, solid, 

 swollen, equivalve, sides rounded, posterior side 

 short, anterior elongately extended ; whitish, par- 

 tially covered with a thick brown silken epidermis ; 

 radiately ribbed, ribs thirty in number, smooth ; 

 area of the ligament rather wide, bent inwards ; 

 umbones curiously contorted in the reverse di- 

 rection. 

 Reeve, Pro. Zool. Soc, 1844. 



Hab. ? 



This is another contrary or reversed species, and a 

 very remarkable one, the shell being completely reversed, 

 whilst the position of the ligamentary area remains the 

 same. The Area contraria and reversa (PI. I. Sp. 5.) 

 are the only species of the genus I have as yet seen ex- 

 hibiting this peculiarity of growth. 



