ARC A. —Plate IX. 



posterior side rather compressly expanded ; whitish, 

 covered with a thin, horny, hluish green cuticle and 

 a brown epidermis ; radiately ribbed, ribs .six or 

 seven and thirty in number, rather flat, obsoletely 

 crenulated towards the umbones ; umbones some- 

 what approximated ; area of the ligament narrow, 

 bent inwards. 



Reeve, Pro. Zool. Soc, 1344. 



Hub. ? 



The nearest approach to the Arca incequivalvis, but a 



shell of more compressed growth ; the valves exhibit a 



still greater disparity of size, and the ribs are rather 



more numerous. 



Species 60. (Mus. Metcalfe.) 

 Arca antiquata. Area testa abbreviato-ovatd, crassd, 

 valde gibbosd, aqirivalvi, lateribus superne angulatis, 

 antico brevi, rotundato, postico subangulato-extenso ; 

 albidd, maeitlis perpaucis sparsis fuscescentibus pos- 

 tice ornatd, epidermide fused, inter costas setosd, in- 

 dutd ; radiatim costatd, costis ad tricenas, angustis, 

 distantibus, acutangularibus, a?iticis prope marginem 

 sulco divisis, umbones versus noduloso-ereimtis ; liga- 

 menti area latiusculd. 

 The ancient Ark. Shell shortly ovate, thick, very 

 gibbous, equivalve, sides angulated at the upper 

 part, anterior side short, rounded, posterior some- 

 what angularly extended ; whitish, ornamented on 

 the posterior side with a few scattered light brown 

 spots, covered with a brown epidermis that is 

 bristly between the ribs ; radiately ribbed, ribs 

 about thirty in number, narrow, distant, sharp- 

 angled, anterior ribs divided by a groove near the 

 margin, nodulously crenated towards the umbones ; 

 area of the ligament rather broad. 

 Linn.eus, Lamarck, Anim. sans vert. (Deshayes' edit.), 

 vol. vi. p. 470. 



Hob. ? 



This shell I take to be the original Area antiquata of 

 Linnaeus ; and it is essentially distinct from the Arcce 

 Deshayesii, secticostata, and others which have been con- 

 founded together under the same common title. 



The light brown spots which are scattered about the 

 posterior side of the Area antiquata are apparently of 

 insignificant character, but it is important to notice that 

 out of the sixty-eight species of unattached Area which 

 have now come under my observation, the Area maculosa 

 is the only other species in which any trace of them 

 may be discovered. 



rotundato, postico elongato-extenso, subattenuato ; 

 alba, epidermide fused ho/osericd indutd ; radiatim 

 costatd, costis niimcrosissimis, tribus vel quatuor et 

 quadraginta, plants, latiusculis, ereberrimis, ante- 

 rioribus sulco divisis, subtilissime ereuii/atis ; umbo- 

 nibus latis, subapproximatis ; ligamenti ared angustd, 

 declivi. 



The close-ribbed Ark. Shell elongately square, equi- 

 valve, sides angulated at the upper part, anterior 

 rounded beneath, posterior angularly extended, a 

 little attenuated ; white, covered with a brown vel- 

 vety epidermis ; radiately ribbed, ribs very nume- 

 rous, three or four and forty in number, flat, rather 

 broad, very close-set, the more anterior ribs di- 

 vided by a groove, very finely crenulated; um- 

 bones broad, rather approximated ; area of the 

 ligament narrow, bent inwards. 



Reeve, Pro. Zool. Soc, 1844. 



Hub. ? 



The ribs of this species are very characteristic, being 



comparatively broad, flat, very close-set, and more in 



number than in any other of the genus. 



(Mus. Cue 



Species 61. (Mus. Cuming.) 

 Arca crebkicostata. Arca testa elongato-quadratd, 

 aquivalvi, lateribus superne angulatis, antico infra 



Specit 



Arca hians. Arca testa elongato-ovatd, cequivalvi, antice 

 hiante, lateribus rotunda/is, postico attenuato-ex- 

 tenso ; albidd, fiiset'scciite pallidc timid, epidermide 

 fused., inter costas squamosa, indutd ; radiatim cos- 

 tatd, cos/is dualats vel tribus el triijinta, latiusculis, 

 p/auu/alis, autieis subobsolete erenulatis, sulco latis- 

 simo divisis ; umbo/iibus subapproi imatis ; ligamenti 

 ared angustd, profundi declivi. 



The gaping Ark. Shell elongately ovate, equivalve, 

 gaping anteriorly, sides rounded, the posterior at- 

 tenuately extended ; whitish, palely stained with 

 light brown, covered with a soft brown epidermis, 

 scaly between the ribs ; radiately ribbed, ribs two 

 or three and thirty in number, rather broad, flat, 

 the anterior ribs rather obsoletely crenulated, di- 

 vided by a very broad groove ; umbones somewhat 

 approximated ; area of the ligament narrow, deeply 

 bent inwards. 



Reeve, Pro. Zool. Soc, 1844. 



Hub. ? 



The shape of the Arca hians approaches somewhat to 

 that of the Arca eymbuformis, but the species differ 

 most essentially from each other on examination. In 

 the Arca hians the valves are equal, the anterior ribs 

 are divided by an unusually broad groove, and the shell 

 gapes at the anterior end to the extent of about three- 

 sixteenths of an inch, a peculiarity of which I have not 

 observed the slightest indication in any other species 

 of this division of the Area:. 



