490 Zoological Society. 



divisis ; ligamenti ared angustd ; umbonibus latis, peculiariter de- 

 presso-vellicatis. 

 Conch. Icon. Area, pi. 5. f. 33. 



Hab. ? 



This is a very remarkable shell, both on account of the large 

 number of ribs, and of the peculiar compression of the umbones. 

 Arca gambiensis. Area testd elongato-quadratd, tenuiculd, valde 

 compressdy in medio coarctatd, cequivalvi, lateribus rotundatis, 

 postico subangulato extenso ; albidd, epidermide tenui olivaced, 

 sparsim setigerd, indutd ; radiatim costatd, costis duabus vel tribus 

 et viginti, Icevigatis, plano-expansis, lira exili intercurrente ; liga- 

 menti ared parvd, angustd ; umbonibus angulato-mucronatis. 

 Conch. Icon. Area, pi. 6. f. 36. 



Hab. Mouth of the Gambia, Africa (found in sandy mud in deep 

 water). 



The ribs of this shell become peculiarly flattened and spread out 

 towards the margin, and there is a fine ridge running down the cen- 

 tre of the intermediate grooves. This is the only species in which I 

 have as yet observed any kind of fine interribbing. 



Arca cuneata. Arca testd oblique ovatd, cequivalvi, marginem 

 versus coarctatd, lateribus superne angulatis, infra oblique rotun- 

 datis ; albidd, radiatim costatd, costis ad vicenas, rotundatis, irre- 

 gular iter nodosis ; ligamenti ared latissimd, declivi ; umbonibus 

 parvis, distantibus. 

 Conch. Icon. Arca, pi. 6. f. 37. 

 Hab. Zanzibar. 



The great width of the ligamentary area, separating the umbones 

 asunder to a considerable extent, imparts a wedge-like form to this 

 shell, by which it may be easily recognised. 



Arca secticostata. Arca testd elongato-ovatd, gibbosissimd, tenui- 

 culd, lateribus superne angulatis, infra suboblique rotundatis ; al- 

 bidd, fuscescente partim tinctd ; radiatim costatd, costis numerosis, 

 angustis, ad quadragenas, quamplurimis sulco subprofundo divisis ; 

 ligamenti ared elongatd, latiusculd; umbonibus tumidis. 

 Conch. Icon. Arca, pi. 6. f. 38. 



Hab. ? 



This shell is probably one of those that have been confounded 

 with the Arca antiquata ; it differs in having a much greater number 

 of ribs, and a larger proportion of them more distinctly grooved. 

 Arca ferruginea. Arca testd ovato-oblongd, cequivalvi, lateribus 

 rotundatis ; albidd, epidermide ferrugineo -fused indutd ; radiatim 

 costatd, costis ad septem et viginti, angustis, compressis, subtilis- 

 sime nodulosis ; ligamenti ared subangustd ; umbonibus oblique in- 

 curvis. 

 Conch. Icon. Arca, pi. 6. f. 39. 



Hab. ? 



The ribs of this species are unusually narrow, and being finely 

 nodulous, present a kind of beaded appearance. 



Arca radiata. Arca testd ovato-oblongd, cequivalvi, lateribus ro- 



