ARCA.— Plate XIII. 



keeled, keel serrated, running down from the um- 

 bone to the margin ; white, faintly stained with 

 pinkish brown ; radiately striated, striae raised, 

 those of the area of the left valve nearly obsolete ; 

 umbones minute, approximated ; area of the liga- 

 ment very long, very narrow, bent inwards. 

 Linnjsus, Syst. Nat. (Gmelin's edit.), p. 3305. 

 Hab. Singapore and Malacca (found in coarse sand and 

 gravel at the depth of from seven to ten fathoms) ; 

 Cuming. 

 This shell cannot fail to be recognised by the extra- 

 ordinary manner in which it is twisted on its cardinal 



Species 87. (Mus. Cuming.) 



Arca cuneata. Area teste! elongato-oblongd, lateribus 

 superne angulatis, ant'tco brevi, rotundato, postico 

 elongato, angulato, carina ab umbone ad marginem de- 

 eurrente, margine ventrali bysso hiante; pal tide fused, 

 epidermide molli lamellatd indutd; radiatim striatd, 

 striis 1 1 /<rat is, further granulosis, ared postico. subin- 

 distinet'e nigricostatd ; umbonibus subapproximatis, 

 ligamenti ared latiusculd, concavd, sulcis ligamenta- 

 riis anticis postieisque, subdistantibus. 



The wedge-like Ark. Shell elongately oblong, sides 

 angulated at the upper part, anterior side short, 

 rounded, posterior elongated, angulated, with a keel 

 running down from the umbone to the margin, 

 ventral margin gaping for the passage of the byssus ; 

 pale brown, covered with a soft thickly laminated 

 epidermis ; radiately striated, striae raised, strongly 

 granulated, posterior area rather indistinctly marked 

 with blackish ribs ; umbones somewhat approxi- 

 mated ; area of the ligament rather broad, concave, 

 ligamentary grooves both anterior and posterior to 

 the umbones, rather distant from each other. 



Reeve, Pro. Zool. Soc, 1S44. 



Hab. Zanzibar (found under stones at low water) ; Thorn. 



The sculpture of this species corresponds precisely 

 to that of the Arca mutabilis ; the form of the shell is 

 more depressly elongated, the anterior side is shorter, 

 and there are ligamentary grooves on the posterior part 

 of the cardinal area as well as on the anterior. 



Species 88. (Mus. Cuming.) 

 Arca alternata. Ai-ea testd oblongd, subcylindraced, 

 medio coarctatd, antice valde attenuatd, lateribus 

 superni obtuso-angulatis, latere an/ico parvo, brevis- 

 simo, postico elongalo, costis duabus validie ab um- 

 bone ad marginem decurrentibus, margine ventrali 

 bysso paululiim hiante; luteo-fuscescente, epidermide 

 peculiuriter crislutd, sparsim indutd; radiatim costa- 

 td, costis mediants planiuseulis, Icevibus, creberrimis. 



sulco medio divisis, costis anticis postieisque validis, 

 crenulatis et striatis, interstitiis profundi sulcatis ; 

 umbonibus subapproximatis ; ligamenti ared postice 

 lanceolato-acuminatd. 

 The interlocking Ark. Shell oblong, somewhat cy- 

 lindrical, contracted in the middle, very much at- 

 tenuated at the anterior end, sides obtusely angled 

 at the upper part, anterior side small, very short, 

 posterior elongated, with two strong ribs running 

 down from the umbone to the margin, ventral mar- 

 gin a little gaping for the passage of the byssus ; 

 yellowish brown, sparingly covered with a pecu- 

 liarly tufted epidermis; radiately ribbed, middle 

 ribs flat, smooth, very close-set, divided down the 

 centre by a groove, anterior and posterior ribs very 

 strong, crenulated and striated, interstices deeply 

 grooved ; umbones somewhat approximated ; area 

 of the ligament posteriorly lanceolately acuminated. 

 Byssoarca alternata, Sowerby, Pro. Zool. Soc, 1833. 

 Hab. West Columbia (found attached to stones, on a 

 rocky bottom, at the depth of twelve fathoms) ; 

 Cuming. 

 This is altogether a very curious and interesting 

 species ; the ribs at each end of the shell have a remark- 

 ably characteristic appearance, and the epidermis, which 

 is disposed in minute isolated tufts, is of quite a novel 

 structure. 



Species 89. (Mus. Cuming.) 

 Arca semitorta. Area testd elliptico-oblongd, semi- 

 tortd, intequivalvi, lateribus rotundatis, antico valde 

 breviore ; albidd, fuscescente pa/lidissimi tinctd, epi- 

 dermide crassd molli indutd ; radiatim striata, striis 

 elevatis, striis longitudinalibus minutis, subtilissime 

 cancellatis ; umbonibus approximates, depresso-ineur- 

 vis ; ligamenti ared elongatd, angustissimd, declivi .■ 

 intus croceo pal/id'e tinctd. 

 The half-twisted Ark. Shell elliptically oblong, 

 half- twisted, inequivalve, sides rounded, anterior 

 side much the shorter ; whitish, very faintly stained 

 with light brown, covered with a thick soft epi- 

 dermis ; radiately striated, striae raised, very finely 

 cancellated with longitudinal striae ; umbones ap- 

 proximated, depressly incurved ; area of the liga- 

 ment long, very narrow, bent inwards ; interior 

 faintly stained with yellow. 

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



p. 460. 

 Hab. San Nicolas, island of Zebu, Philippines (found in 

 sandy mud at the depth of six fathoms) ; Cuming. 

 This is a shell of much more solid growth than the 

 Arca tortuosa ; it turns upon its cardinal axis in the 

 same direction as that species, but (as its name signifi- 

 cantly implies) it is twisted to only one half the extent, 



