ARCA.— Plate I. 



setigerd, obtectd ; radiatim costatd, costis angustis | 

 numerosis, duabus vel tribus et quadraginta, marginem 

 versus leviter canaliculatis ; ligamenti ared suban- 

 gustd. 



The stout Ark. Shell ovate, inequivalve, rather thin, 

 ventricose, anterior side rounded, posterior some- 

 what angular and stretched out ; white, covered over 

 with a hrown epidermis, which is bristly in the in- 

 terstices between the ribs; radiately ribbed, ribs 

 narrow, numerous, two or three and forty in num- 

 ber, slightly canaliculated towards the margin ; area 

 of the ligament rather narrow. 



Sowerby, Pro. Zool. Soc, 1833. 



Hab. Atacamas, West Columbia (dredged from the depth 

 of seven fathoms) ; Cuming. 



Chiefly remarkable for its great : 



iber of ribs. 



Species 4. (Mus. Cuming.) 



Arca grandis. Area testa subtequivalci, quadratd, solidd, 

 lateribus superne subauriculatis, infra rotundatis, 

 postico vix extenso ; radiatim costatd, costis rotun- 

 datis, quatuor vel quinque et viginti, anticis leviter 

 nodulosis ; umbonibus recti- incurvis ; costis caruleo- 

 viridibus, iiiterstitiisall/idis, epidermide corned scabrd 

 indutis. 



The great Ark. Shell nearly equivalve, square, 

 solid, sides slightly auriculated at the upper part, 

 rounded beneath, posterior side a little extended; 

 radiately ribbed, ribs rounded, four or five and 

 twenty in number, anterior ribs slightly nodulous ; 

 umbones straightly incurved ; ribs dark bluish 

 green, interstices whitish, covered with a rough 

 horny epidermis. 



Broderip and Sowerbt, Zool. Journ., vol. iv. 



Hab. Heal Llejos, Bay of Guayaquil, &c, western coast 

 of South America ; Cuming, Hinds. 



A large example of this species has been selected. 



Species 5. (Mus. Cuming.) 



Arca reversa. Arca testa obliqud, cequivalvi, turyidd, 

 latere postico rotundato, antico oblique depresso- 

 truncato ; radiatim costatd, costis ad quadragenas, 

 confertis, planulatis, subtilissime rugiilosis; albd, 

 epidermide molli nigricante indutd ; umbonibus ap- 

 proximates ; ared subnulld. 



The reversed Auk. Shell oblique, equivalve, turgid, 

 posterior side rounded, anterior obliquely depressly 

 truncated ; radiately ribbed, ribs about forty in 

 number, close-set, flat, very finely wrinkled ; white, 



covered with a soft blackish epidermis ; umbones 

 approximated ; ligamentary area very small. 



Gray, Sowerby, Pro. Zool. Soc, 1833. 



Hab. Tumbez, Peru (found in soft mud at the depth of 

 seven fathoms) ; Cuming. 



The Arca reversa may be distinguished by its peculiar 

 growth, and by the flat, finely wrinkled surface of the ribs. 



Species 6. (Mus. Cuming.) 



Arca brevifrons. Arca testa oblongd, inccquivalvi, la- 

 ti re antico brevi, rotundato, postico longiusculo, la- 

 tiore, superne angulato ; albd, virideseente tinctd 

 epidermide fused, inter castas setigerd, indutd; r< 

 diatim costatd. costis duabus vel tribus et viginti, ph 

 nulatis, casus marginem in medio suleo divisis ; un 

 bonibus appro.rimatis ; area nulla. 



The short-fronted Ark. Shell oblong, inequivalve 

 anterior side short, rounded, posterior rather long 

 wider, angulated at the upper part ; white, stained 

 with light green, covered with a brown epidermis 

 which is bristly between the ribs ; radiately ribbed 

 ribs two or three and twenty in number, flat, divided 

 in the middle towards the margin by a groove 

 umbones approximated ; no ligamentary area. 



Sowerby, Pro. Zool. Soc, 1833. 



Hub. Tumbez, Peru (dredged from soft mud at the depth 

 of seven fathoms) ; Cuming. 

 This shell answers very nearly to Lamarck's,-//w Indica 



which I have not as yet succeeded in identifying. 



Species 7. (Mus. Cuming.) 



Arca labiata. Arca testd subquadrato-globosd, valde 

 inccquicalvi , latere antico breviore, rotundato, postico 

 subungulato ; albd, epidermide virideseente-fuscd, ad 

 anticam sulcorum partem setigerd, indutd ; radiatim 

 costatii. costis anticis, valeic sinistra/is prweipue, 

 subtiliter rugiilosis, costis posticis Itcvibus, latioribus ; 

 umbonibus remolis ; ligamenti ared lata, r/iumlioidcd. 



The litped Ark. Shell somewhat squarely globose, 

 very inequivalve, anterior side shorter, rounded, 

 posterior rather angular ; white, covered with a 

 brown epidermis, which is bristly on the anterior 

 side of the grooves ; radiately ribbed, anterior ribs, 

 of the left valve especially, finely wrinkled, poste- 

 rior ribs smooth, wider ; umbones remote from each 

 other ; area of the ligament wide, diamond-shaped. 



Sowerby, Pro. Zool. Soc, 1833. 



Hab. Real Llejos and Tumbez, Peru (dredged from 

 mud at the depth of seven fathoms) ; Cuming. 

 An interesting and well-defined species. 



