ARCA.— Plate V 



This is a light inflated shell of rather large size, at 

 present unique in the collection of Mr. Cuming. 



Species 31. (Mus. Cuming.) 

 Arca cymbjeformis. Area testa elongato-ovatd. tumi- 

 diusculd, ituequivalvi, posticr attenuatd; albidd, epi- 

 dermide corned viridescente, inter costas squamosa, 

 indutd; radiatim costatd, costis duabus et triginta, 

 valvte sinistralis rotundatis, tuberculato-crcnatis, val- 

 var dextralis planulatis, anticis tantum crenatis ; liga- 

 menti area parvd, declivi. 

 The boat-shaped Ark. Shell elongately ovate, rather 

 swollen, inequivalve, attenuated posteriorly ; white, 

 covered with a green horny epidermis, scaly be- 

 tween the ribs; radiately ribbed, ribs two and 

 thirty in number, those of the left valve rounded 

 and tuberculously crenated, of the right valve the 

 anterior only crenated ; area of the ligament small, 

 bent inwards. 

 Reeve, Pro. Zool. Soc, 1844. 



Hab. ? 



This is a shell of very peculiar growth, inasmuch as 

 the posterior side is smaller and more attenuated than 

 the anterior, a condition reverse to that usually pre- 

 sented. 



Species 32. (Mus. Cuming.) 

 Arca Japonica. Arca testd elongato-ovatd, tenuiculd 

 valde inaquivalvi, lateribus rotundatis, postico latiore, 

 antico brevissimo ; alba, epidermide corned fuscescente, 

 inter costas squamosa, indutd ; radiatim costatd, 

 tis quinque et triginta, angustis, Itevibus, planulatis 

 anticis subobsolete nodulosis ; ligamenti ared angus 

 tissimd. 

 The Japan Ark. Shell elongately oval, rather thin, 

 very inequivalve, sides rounded, posterior side the 



wider, anterior very short ; white, covered with a 

 light brown horny epidermis, scaly between the 

 ribs ; radiately ribbed, ribs five and thirty in num- 

 ber, narrow, smooth, flat, the anterior rather ob- 

 soletely nodulous ; area of the ligament very nar- 

 row. 



Reeve, Pro. Zool. Soc., 1844. 



Hab. Coast of Japan (found in sandy mud) ; Dr. Sie- 

 bald. 

 The lateral extremities of this shell are peculiarly 



rounded. 



Species 33. (Mus. Metcalfe.) 

 Arca vellicata. Arca testa trapeziformi, subcompres- 

 sd, in medio leviter contractu, vix aquivalvi, lateri- 

 bus superne anguhtis, antico infrd rotunda to, postico 

 angulato-expanso ; albidd, epidermide, inter costas 

 squamosa, partim indutd ; radiatim costatd, costis 

 angustis, numerosissimis, quinquagenis aut pluribus, 

 anticis sulco divisis ; ligamenti ared angustd; umboni- 

 bus latis, peculiariter depresso-vcUicatis. 

 The pinched Ark. Shell trapeziform, a little com- 

 pressed, slightly contracted in the middle, nearly 

 equivalve, sides angulated at the upper part, ante- 

 rior side rounded beneath, posterior angularly ex- 

 panded ; whitish, partially covered with an epi- 

 dermis which is scaly between the ribs ; radiately 

 ribbed, ribs narrow, very numerous, upwards of 

 fifty in number, anterior ones divided by a groove ; 

 area of the ligament narrow ; umbones wide, pecu- 

 liarly depressly pinched. 

 Reeve, Pro. Zool. Soc, 1844. 



Hab. ? 



This is a very remarkable shell, both on account of 

 the large number of ribs, and of the peculiar compres- 

 sion of the umbones. 



