ARC A. 



Plate XII. 



Species 76. (Mus. Cuming.) 



Arca Lithodomus. Arca testa longissimd, teiiaiet/ld, an- 

 gustd, cylindraceo-cuneiformi, decussato-striatd, sub- 

 tilissime et concinrie imbricato-squamed, ut plurimum, 

 umbon.es versus prcecipue, erosd ; latere antico brevis- 

 simo, oblique trimcato, carina acu/d setigerd ab urn- 

 bone ad marginem decurrente ; postico longissimo, ex- 

 tremitatem versus compresso, rotundato-acuminato, 

 margine ventruli bysso paululhm hiante ; ligamenti 

 area angustissimd, profundi dec/ivi, antice obtuso- 

 angulatd, postice acuminata, corio corneo nigrieante 

 indutd ; sulcis ligamentariis ad duodecim, obliquis, 

 subdistantibus. 

 The Litiiodomcs-like Ark. Shell very long, rather 

 thin, narrow, cylindrically wedge-shaped, decus- 

 sately striated, very finely and beautifully imbri- 

 catcly scaled, but for the most part eroded, especially 

 towards the umbones ; anterior side very short, 

 obliquely truncated, with a bristly keel running 

 from the umbone to the margin; posterior very long, 

 compressed towards the extremity, rotundately acu- 

 minated, ventral margin a little gaping ; area of 

 the ligament very narrow, deeply bent inwards, 

 anteriorly obtusely angled, posteriorly acuminated, 

 covered with a black horny cuticle ; ligamentary 

 grooves about twelve in number, oblique, rather 

 distant. 



Byssoarca Lithodomus, Sowerby, Pro. Zool. Soc, 

 1S33. 

 Hub. Monte Christo, West Columbia (found in stones 

 at low water) ; Cuming. 

 This extraordinary species of Area most probably be- 

 comes imbedded in stones and masses of rock by the aid 

 of some powerful solvent secretion, like the Lithodomi 

 and other terebrating mollusks. 



Species 77. (Mus. Cuming.) 

 Arca obtusa. Area testa oblongd, Modioheformi, la- 

 teribus obtuso-rotundatis, margine ventrali bysso pau- 

 lulum hiante; alba, epidermide nigrieante subsquu- 

 mosd partim indata ; radiatim striata, striis elevatis ; 

 umbonibus depressiusrulis, approximatis ; ligamenti 

 area pared, ungustd, profundi declivi. 

 The obtuse Ark. Shell oblong, Modiola-shaped, sides 

 obtusely rounded, ventral margin a little gaping; 

 white, partially covered with a rather scaly blackish 

 epidermis ; radiately striated, stria? raised ; umbones 



rather depressed, approximated ; area of the liga- 

 ment small, narrow, deeply bent inwards. 

 Reeve, Pro. Zool. Soc, 1844. 

 Hab. Coast of Japan (under stones) ; Dr. Siebald. 



Very like a Modiola in shape, but not the recent ana- 

 logue of the fossil A. ModioUcformis of Deshayes. 



Species 7S. (Mus. Cuming.) 

 Arca illota. Area testa abbreviato-ovatd, ventraliter 

 compressd, subcuneiformi, lateribus superne vald'e at- 

 tenuatis, margine ventrali bysso vix hiante ; albidd, 

 epidermide tenui, peculiariter foliaeed, indutd ; ra- 

 diatim striatd, striis rudibus, elevatis, striisaliis di- 

 stantioribus subirregulariter decussatis ,• umbonibus 

 grandibus, compresso-approximatis ; ligamenti area 

 angustd, minutd.fer'e occulta. 

 The unwashed Ark. Shell shortly ovate, ventrally 

 compressed, somewhat wedge-shaped, sides very 

 much attenuated at the upper part, ventral mar- 

 gin scarcely gaping ; whitish, covered with a thin, 

 peculiarly foliaceous epidermis ; radiately striated, 

 striae rude, raised, somewhat irregularly crossed by 

 other more distant strise; umbones large, compressly 

 approximated ; area of the ligament narrow, very 

 small, almost concealed. 



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

 Hab. Gulf of Nicoiya, Central America (found under 

 stones) ; Cuming. 

 This species is of very peculiar shape, and has a cu- 

 rious light foliaceous epidermis, extending over the ven- 

 tral margin, like the horny epidermis of the Solemya. 



Species 79. (Mus. Cuming.) 

 Arca velata. Area testd ovato-oblonga, curviitsculti, 

 subcompressd, lateribus superne leviter angulatis, in- 

 fra rotundatis, latere antico parvo, brevissimo, pos- 

 tico latiore, margine ventrali bysso hiante ; alba. 

 epidermide fused, peculiariter squamosd, versus mar- 

 ginem indutd, squaniis aruminalis ; radiatim strinl.i 

 striis fortibus, elevatis, nodulosis ; ligamenti area 

 angustd, declivi ; sulcis ligamentariis numerosis, con- 

 fertis. 

 The clothed Ark. Shell ovately oblong, somewhat 

 compressed, rather curved, sides angulated at the 

 upper part, rounded beneath, anterior side small, 

 very short, posterior much wider, ventral margin 

 gaping ; white, covered towards the margin with a 



March 1844. 



