ARCA.— Plate III. 



Hab. Basey, island of Samar, Philippines (found in 

 sandy mud at low water) ; Cuming. 

 The horny covering of this species is of an unusually 

 transparent character. 



Arca 



I ii 



Species 17. (Mus. Cuming.) 

 rasiliana. Arca testa subovali, tenui, ventri- 

 cosd, valde incequiva/vi, lateribus superrie rotundatis, 

 attematis, postico subtus angulato ; albidd, rufescente 

 pallidissime tinctd, epidermide subnulld ; radiatim 

 costatd, costis ad sex et viginti, planulatis, nonnullis 

 subobsoktis, anticis ruguloso-crenatis, posticis Icsvi- 

 bus, interstitiis valves sinistralis angustissimis, dex- 

 tralis latiusculis ; ligamenti ared parvd, latiusculd. 

 Brazilian Ark. Shell somewhat oval, thin, ven- 

 tricose, very inequivalve, sides rounded and atte- 

 nuated at the upper part, posterior angulated be- 

 neath ; whitish, very palely stained with light rusty 

 red, epidermis almost wanting ; radiately ribbed, 

 ribs about twenty- six in number, flat, some of them 

 almost obsolete, anterior ribs crenated with fine 

 wrinkles, posterior smooth, interstices between the 

 ribs very narrow on the left valve, rather broad on 

 the right ; ligamentary area small, but rather wide. 

 Lamarck, Anim. sans vert. (Deshayes' edit.), vol. vi. 

 p. 473. 



Arca Cardiiformis, Sowerby. 

 Hab. Rio Janeiro, coast of Brazil ; Lamarck. San Bias, 

 Bay of California (found on the sands) ; Cuming. 

 The Arca Brasiliana figured by Sowerby in his ' Ge- 

 nera of Shells' for the Arca huequivalvis, and subse- 

 quently described by the same author in the ' Proceedings 

 of the Zoological Society' for 1833 as a new species, is 

 a shell of very peculiar character, inasmuch as the ribs 

 are wider in the left valve than they are in the right ; it 

 is moreover curiously attenuated towards the hinge, and 

 presents, as Mr. Sowerby remarks, the appearance of a 

 common Cockle. 



Species 18. (Mus. Cuming.) 



Arca tuberculosa. Arca testd subelongato-ovatd, tur- 

 gidd, obliqud, esquivaivi, lateribus superne subauricu- 

 latis, infra rotundatis, postico oblique extenso ; albidd, 

 epidermide erassii uiqricuute-fusai obteetd ; radiatim 

 costatd, costis sex velseptem et triginta, angustis, con- 



fertis, versus marginem sparsim tubereuliferis, anticis 

 prasertim ; ligamenti area angustd, umboiiibus sub- 

 compressis. 

 The tubercled Ark. Shell rather elongately ovate, 

 swollen, oblique, equivalve, sides slightly auricu- 

 lated at the top, rounded beneath, posterior side 

 obliquely stretched out ; whitish, covered over with 

 a thick blackish brown epidermis ; radiately ribbed, 

 ribs six or seven and thirty in number, narrow, 

 close-set, armed with scattered tubercles towards 

 the margin, especially the anterior ones ; area of 

 the ligament narrow, umbones a little compressed. 

 Sowerby, Pro. Zool. Soc, 1833. 



Hab. Real Llejos, Central America (found at low water 

 at the roots of the mangrove trees) ; Cuming. 

 This is a very characteristic species, and may be re- 

 coguiseil by a feature which I have not yet observed in 

 any other species of the genus ; namely, that of having 

 a few scattered peculiar-looking tubercles towards the 

 margin. There is not the slightest indication of any 

 tubercles on the umbonal portion of the shell. 



Species 19. (Mus. Cuming.) 

 Arca lobata. Arca tcstii ovali, cequivalvi, lateribus su- 

 perm vix angulatis, infra rotundatis, latere antico 

 brevissimo, postico compresso, latissinie lobato; albidd, 

 fuscescente pallide tinctd, epidermide crassd, subsqua- 

 mosd obteetd ; radiatim costatd, costis ad sex et vi- 

 ginti, obliquis, subnodosis, posticis latioribus, prope 

 marginem subobsolete expansis ; ligamenti ared me- 

 rliocri. 

 The lobed Ark. Shell oval, equivalve, sides slightly 

 angular at the upper part, rounded beneath, an- 

 terior side very short, posterior compressed, very 

 widely lobed ; whitish, covered with a thick rather 

 scaly epidermis ; radiately ribbed, ribs about six and 

 twenty in number, oblique, slightly nodulous, pos- 

 terior ribs wider, somewhat obsoletely spread near 

 the margin ; area of the ligament middling. 

 Reeve, Pro. Zool. Soc., 1844. 

 Hab. West Indies ? 



The chief peculiarity of this shell consists in the 

 widely expanded compressed growth of the posterior 

 side ; it is remarkably distinct from any hitherto de- 

 scribed species. 



