A R C A. 



Plate III. 



Species 14. (Mus. Cuming.) 



Aria gubernaculum. Arca testa subelongato-ovatd, 

 eam/iressiiiseii/a, inai/i/ivalvi, lateribus superne an- 

 gulatis, latere antico valdi breviore, postico com- 

 presso-expanso ; viridescente, epidermide inter costas 



s</niim(isii imlutii ,- niilintiiii rostatd, costis duabus 

 vel tribus rt triijintn, plannlatis, Ivvitrr noduloso- 

 serratis ,- ligamenti area angustd. 



The rudder Ark. Shell somewhat elongately oval, 

 rather compressed, inequivalve, sides angulated ;it 

 tlu upper part, anterior side very much the shorter, 

 posterior compressly expanded ; greenish, covered 

 with an epidermis, which is scaly between the ribs ; 

 radiately ribbed, ribs two or three and thirty in 

 number, flat, slightly nodulously serrated ; area of 

 the ligament narrow. 



Reeve, Pro. Zool. Soc, 1844. 



/lab. Basey, island of Samar, Philippines (found in 

 coarse sand and amongst shells at the depth of four 

 fathoms) ; Cuming. 



This shell may be recognised by the compressly ex- 

 panded character of the posterior side. 



Specie? 15. (Fig. a and b, Mus. Cuming.) 



Arca granosa. Area testa suborbieulari-ovatd, ventri- 

 ■ :<:, aquivaivi, lateribus supent'e vi.e niii/u/alis, 

 infra rotitialatis ; albiihi, epidermide corned lavi in- 

 dutii ; railiiifini rostn/ii, rostis ail viginti, tuberculato- 

 crenatis, marginem versus /state subevanidis ; liga- 

 menti area auijusliuset/ld. 



The granuxous A rk . Shell somewhat orbicularly ovate, 

 rather ventricose, equivalve, sides slightly angu- 

 lated at the upper part, rounded beneath ; whitish, 

 covered with a smooth horny epidermis ; radiately 

 ribbed, ribs about twenty in number, tuberculously 

 crenated, less distinctly developed towards the mar- 

 gin in adult examples ; area of the ligament rather 

 narrow. 



L.INN.EUS, Syst. Nat., p. 1 142. 



Lamarck (A. granosa, var. b and c), Anim. sans vert., 

 vol. vi. p. 472. 



llab. Philippine Islands, (see. 



The Arca rkombea (Sp. 12) appears to have been 

 very much confounded by authors, myself amongst the 



number, (Conch, Syst., vol. i. PI. LXXII. fig. 3, from 

 Sowerby's ' Genera of Shells'.) with the species under 

 consideration; and the cause may be fairly traced to 

 Lamarck. Three distinct varieties of Arca granosa are 

 specified by that writer as follows ; — var. a, ribs twenty- 

 five or twenty-six in number, with the umbones large ; 

 var. /), ribs eighteen to twenty in number, with the um- 

 bones ratherremote ; and var. r. ribs eighteen to twenty 

 in number, with tin- rihs more approximated. Now, from 

 tin- specific importance my observations lead me to attach 

 to the number of the ribs, I feel no hesitation in saying 

 tli.it Lamarck's Area granosa, var. a, is an Arca rhombea, 

 which species has twenty-six ribs with large umbones. 

 The varieties b and c are unquestionably the true species, 

 but the differences noted between them are nothing 

 more than the common indications of growth. All Arca 

 in a young state have the umbones more approximated 

 than in an advanced state of growth ; as the ligamentary 

 area increases with age, so the umbones recede from each 

 other. 



The Area granosa, it must be observed, is of an oval 

 convex form, with the umbones rather depressed, though 

 full anil round ; the Arca rhombea, on the other hand, is 

 of a gibbous quadrilateral form, with the umbones re- 

 markably prominent, and the ribs not so strongly tu- 

 bercled. 



Spi cies lb'. (Mus. Cuming.) 

 Arca cornea. Area testa subijiiai/riltd, i/ililiasiiiseiiiii, 



intequivalvi, lateribus superne angulatis, infra rotun- 



datlS, latere iinfieu breeinre; albd, COrio trnn't, cor- 

 nea, viriilesernle, iphleriniite inter castas squamosd 



inilitld ; radiatim costatd, east is ad novem ei viginti, 

 vulva- (le.rtrnlis plants, siuistra/is nui/ii/nso-crenatis ,- 

 ligamenti area mediocri. 



The horny Ark. Shell somewhat square, rather gib- 

 bous, inequivalve, sides angulated at the upper 

 part, rounded beneath, anterior side the shorter ; 

 white, covered by a thin horny green coat with 

 an epidermis, which is scaly between the ribs ; ra- 

 diately ribbed, ribs about twenty-nine in number, 

 those of the right valve flat and smooth, of the left 

 valve nodulously crenated ; area of the ligament 

 middling. 



Reeve, Pro. Zool. Soc., 1844. 



January 1844. 



