A U C A. 



Species 8. (Mus. Cuming.) 



Arca i'ihi.a. Arca testa orbiculari, valdi gibbosd, sub- 

 inaquivalvi, vix inaquilaterali, lateribus supernl an- 

 gulatis, infra rotundatis ; albidd, fuscesi 

 tinctii, epidermide fused, inter costas setosd, indutd ; 

 radiatim costatd, coslis quinque vel sex et viginti, au- 

 gust is, valves sinistra/is omnibus, vah a 

 ant ids tan turn, subtiliter nodulosis; umbonibus sub- 

 remotis, ligamenti ared elongato-rhomboided. 



Tin; little ball Ark. Shell orbicular, ver\ i 



slightly inequivalve, nearly inequilateral. Bides an- 

 gulated at the upper part, rounded below ; whitish, 

 palely stained with light brown, covered with a 

 brown epidermis, full of bristles between the ribs ; 

 radiately ribbed, ribs five or six and twenty in 

 number, narrow; all the ribs of the left valve, an- 

 terior ribs only of the right valve, finely nodulous ; 

 umbones rather remote, area of the ligament elon 

 gately diamond-shape. 



Reeve, Pro. Zool. Soc, 1844. 



Hah. Island of lianas, Philippines (found in sandy mud 

 at the depth of six fathoms) ; Cuming. 



This - that the shell is 



as wide as it is high, and it is the same in all stapes and 

 varieties of growth. 



Species 9. (Mus. Metcalfe.) 



\m \ womala. Area testa quadrato-ovatd, subcom- 



inaquivalvi, latere antico rotunda/a, postica 



undato ; albd, viridescente pallide 



m itn indutd .- 



radiatim costatd, costisocto et viginti, value 



planvlatis, sinistralis conmis, iwdulusis ; 

 ligamenti urea angustd. 

 The anomalous Ark. Shell squarely ovate, rather 

 compressed, inequivalve, anterior side rounded, 

 posterior side rather angularly rounded ; white, 

 stained with light green, sparingly covered with a 

 horny epidermis between the ribs ; radiately ribbed, 

 ribs eight-and-twenty in number ; those of the right 

 valve flat and smooth, of the left valve convex and 

 nodulous ; area of the ligament narrow. 

 Reeve, Pro. Zool. Soc, 1844. 

 Hab. ? 



It seems rather anomalous that the ribs in one valve 

 of this shell should be convex and nodulous, whilst in 

 the other they are fiat and smooth ; but it is an interest- 

 ing fact that the larger valve of the inequivalve species 

 of Arca exhibits a much more elaborate style of sculpture 

 than the smaller. The Arca pi/ulv just described shows 

 this remarkable peculiarity very distinctly, but not * u 

 prominentlj as the species under consideration ; and it 

 now remains to be determined whether this difference 

 arises from any superiority of organization in the left 



lobe of the mantle of the animal inhabitant. 



10. (Mus. ('uming.) 



Abca Formosa. Arca testd elongato-ovatd , eequivalvi, 



lateribus supernl angulatis, infra rotundatis, latere 



antico valdi breviore ; albicante, fuscescente pallid', 



tinctii, epidermide fused, nitente, inter castas setosis- 



simd, utitectii ; radiatim costatd. cost is quinqi 



sex it triginta, planulatis, posticis latioribus, anticis 

 subtiliter nodulosis, anlerioribus sulco dirisis ,• liga- 

 menti ared longissimd, latiusculd ; cardine angustis- 

 stum, dentibus pa 



The beautiful Ark. Shell elongately ovate, equivalve, 

 sides he upper part, rounded beneath, 



anterior side much the shorter; whitish, palely 

 stained with light brown, covered over with a brown 

 shining epidermis, which is very long and bristly 

 between the ribs; radiately ribbed, ribs five or six 

 and thirty in number, flat, posterior wider, anterior 

 finely nodulous, the most anterior divided by a 

 groove ; area of the ligament very long and rather 

 broad ; hinge very narrow, teeth small and nume- 

 rous. 



Sowerby, Pro. Zool. Soc, Is;';:. 



Hab. Gulf of Tehuantepec, Mexico (found in sandy mud 

 at the depth of from ten to twelve fathoms) ; Cu- 

 ming. 



No one can fail to recognise this beautiful species by 

 the accurate delineation of it which accompanies this 

 description. Mr. Cuming possesses another specimen 

 almost entirely covered with an epidermis, in which the 

 bristles are at least half an inch long at the posterior 

 end of the shell. 



December 1843. 



