ARCA.— Plate XII. 



peculiarly scaled epidermis, scales acuminated ; ra- 

 diately striated, stria? strong, raised, nodulous ; 

 area of the ligament narrow, bent inwards ; liga- 

 mentary grooves numerous, close-set. 

 Byssoarca velata, Sowerby, Pro. Zool. Soc, 1S33. 



Hab. Lord Hood's and Chain Islands, Pacific Ocean 

 (found attached to shells) ; Cuming. 

 The epidermis of this shell lies at the posterior end 



in thick-set, brittle, horny plates. 



Species 80. (Mus. Cuming.) 



Arca obliquata. Area testd subcuneiformi, cvrvd, 

 medio coaretatd, lateribus rotundatis, latere a?itico 

 parvo, brevissimo, postico latiore, suboblique expanso, 

 margine ventrali bysso vix hiante ; albidd, viri- 

 descente tinctd, epidermide fused acuminato-squamosd 

 versus marginem indutd ; radiatim striatd, striis 

 elevatis, 7iumerosis, confertis ; umbonibus approxi- 

 matis ; ligamenti area angustd. 



The crooked Ark. Shell somewhat wedge-shaped, 

 curved, contracted in the middle, sides rounded, 

 anterior side small, very short, posterior much 

 wider, rather obliquely expanded, ventral margin 

 scarcely, if at all, gaping ; white, stained with 

 green, covered towards the margin with a brown 

 acuminately scaled epidermis ; radiately striated, 

 striae raised, numerous, close-set ; umbones ap- 

 proximated ; area of the ligament narrow. 



Gray, Wood, Ind. Test. Supp., pi. 2. f. 4. Hanley, 

 Species of Shells, p. 158. 



Hab. Calbayog, island of Samar, Philippines (found un- 

 der stones at low water) ; Cuming. 

 The outline of this shell approaches very nearly to 



that of a figure in the ' Encyclopedie Methodique,' to 



which Lamarck refers for the Arca Magellanica of 



Chemnitz. 



Species 81. (Mus. Cuming.) 

 Arca decussata. Area testd ov at o- oblong d, lateribus 

 supenie subangulatis, infra angulato-rotundatis, latere 

 antico brevissimo, postie'e latiore, subexpanso, mar- 

 gine ventrali bysso paululum hiante ; a/bd, fus- 

 cescente pallidissime tinctd, epidermide fused squa- 

 moso-setosd versus marginem indutd ; decussatim 



striatd, striis radiantibus elevatioribus; umbonibus ap- 

 proximatis ; ligamenti area angustissimd, declivi, fere 

 occulta. 



The decussated Ark. Shell ovately oblong, sides 

 rather anfi'ulated at the upper part, angularly 

 rounded beneath, anterior side very short, posterior 

 wider, rather expanded, ventral margin a little 

 gaping; white, very palely stained with light brown, 

 covered towards the margin with a scaly bristly epi- 

 dermis; decussately striated, radiated stria? the more 

 raised ; umbones approximated ; area of the liga- 

 ment very narrow, bent inwards, almost concealed. 

 Byssoarca decussata, Sowerby, Pro. Zool. Soc, 

 1833. 



Hab. Lord Hood's and Chain Islands, Pacific Ocean; and 

 Philippine Islands (attached to shells) ; Cuming. 



A delicately sculptured shell, in which the epidermis 

 rises from between the striae in scaly bristles. 



Species 82. (Mus. Cuming.) 



Arca fusca. Arca testd ovato-oblongd, lateribus rotun- 

 datis, latere antico brevi, postico subelongato, mar- 

 gine ventrali bysso paululum hiante ; rubido-fuscd, 

 ad umbones albiradiatd. epidermide subtilissime se- 

 tosd sparsim indutd; striis exi/ibus, elevatis, confertis, 

 decussald ; umbonibus approximates; ligamenti ared 

 angustissimd, profund'e declivi. 



The brown Ark. Shell ovately oblong, sides rounded, 

 anterior side short, posterior rather elongated, ven- 

 tral margin a little gaping ; reddish brown, rayed 

 with white at the umbones, sparingly covered with 

 a very fine bristly epidermis ; decussated with deli- 

 cate, close-set, raised stria? ; umbones approxi- 

 mated ; area of the ligament very narrow, deeply 

 bent inwards. 



Bruguiere, Enc. Meth. vers, p. 102. 



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

 p. 466. 



Amygdalum frixum sive tostum, Chemnitz. 

 Arca barbata, var., Gmelin. 



Hab. Island of Mindoro, Philippines (found under stones 

 at low) water, ; Cuming. 

 This species may easily be distinguished from the 



Arca barbata by the white rays which characterize the 



umbones. 



