494 Zoological Society. 



differs from any previously described species in the arrangement and 

 formation of the ribs. 



Arca disparilis. Area testd ovatd, tenui, subcompressd, valde in- 

 cequivalvi, lateribus angulato-rotundatis , latere postico subcom- 

 presso-expanso ; albidd, corio corneo tenui ccEruleo-viridescente, 

 epidermide fused indutd ; radiatim costatd, costis sex vel septem et 

 triginta, planiusculis , umbones versus obsolete erenulatis ; umboni- 

 bus subapproximatis ; ligamenti ared angustd, declivi. 



Conch. Icon. Arca, pi. 9. f. 59. 



Hab. ? 



The nearest approach to the Arca incequivalvis, but a shell of more 

 compressed growth ; the valves exhibit a still greater disparity of size, 

 and the ribs are rather more numerous. 



Arca crebricostata. Arca testd elongato-quadratd, mquivalvi, la- 

 teribus superne angulatis, antico infra rotundato, postico elongato- 

 extenso, subattenuato ; albd, epidermide fused holosericd indutd; 

 radiatim costatd, costis numerosissimis, tribus vel quatuor et qua- 

 draginta, plants, latiusculis, creberrimis, anterioribus sulco divisis, 

 subtilissime erenulatis; umbonibus latis, subapproximatis; liga- 

 menti ared angustd, declivi. 



Conch. Icon. Arca, pi. 9. f. 61. 



Hab. ? 



The ribs of this species are very characteristic, being comparatively 

 broad, flat, very close- set, and more in number than in any other of 

 the genus. 



Arca hians. Arca testd elongato-ovatd, mquivalvi, antice hiante, 

 lateribus rotundatis, postico attenuato-extenso ; albidd, fuscescente 

 pallide tinctd, epidermide fused, inter costas squamosd, indutd ; 

 radiatim costatd, costis duabus vel tribus et triginta, latiusculis, 

 planulatis, anticis subobsolete erenulatis, sulco latissimo divisis ; 

 umbonibus subapproximatis ; ligamenti ared angustd, profundi de- 

 clivi. 

 Conch. Icon. Arca, pi. 9. f. 62. 



Hab. ? ' 



The shape of the Arca hians approaches somewhat to that of the 

 Arca cymbaformis, but the species differ most essentially from each 

 other on examination. In the Arca hians the valves are equal, the 

 anterior ribs are divided by an unusually broad groove, and the shell 

 gapes at the anterior end to the extent of about three- sixteenths of 

 an inch, a peculiarity of which I have not observed the slightest in- 

 dication in any other species of this division of the Arca. 



Arca occlusa. Arca testd subquadratd, valde gibbosd, inaquivalvi, 

 lateribus superne attenuato-angulatis ; albd; radiatim costatd, 

 costis septem vel octo et viginti, subdistantibus , valvce dextralis 

 l&vibus, sinistralis nodulosis ; umbonibus prominentibus , peculiari- 

 ter approximatis ; ligamenti ared subdeclivi. 

 Conch. Icon. Arca, pi. 10. f. 64. 



Hab. ? 



The umbones of this shell are so closely approximated over the 



