46 



the lio-araentary area remains the šame. This and the Arca reversą, 

 Gray, are the only species of the genus I have as yet seen exhibit- 

 ing this peculiarity of growth. 



Arca angicostata. Jrca iestd subguadrato-ovatd, subtEguivalvi, 

 lateribus superne angulatis, infra rotnndatis, antico brevi ; albidd, 

 fuscescente parthn tinctd, epidermide fuscd, inter costas sąuamosd, 

 indutd ; rndiatim costatd, costis ad tricenas, anticis angustis, sub- 

 distantibus, leviter crenulatis, posticis latioribus, expansioribus ; 

 Ugamenti ared latiusculd. 



Conch. Icon. Arca, pi. 9. f. 57. 



Hab. ? 



There is a very unusual disproportion in the width of the ribs of 

 this species, the anterior ribs being exceedingly narrow, whilst the 

 posterior are wider and spread out as it -vvere. 



Arca loricata. Arca testd subguadraid, gibbosd, lateribus superne 

 attenuato-angulatis, antico brevi, rotundato, postico angulato- 

 extenso ; albidd, corio corneo cceruleo-viridescente loricata, epider- 

 mide tenui, fuscd, inter costas squamosd, indutd ; radiatim costatd 

 costis septem vel octo et viginti, planiusculis, acutangulis ; umbo- 

 nibus subapproximatis ; Ugamenti ared leviter decltvi. 



Conch. Icon. Arca, pi. 9. f. 58. 



Hab. ? 



This shell is covered with a peculiarly strong horny cuticle, and 

 difFers from any previously described species in the arrangement and 

 formation of the ribs. 



Arca disparilis. Arca testd ovatd, tenui, subcompressd, valde in- 

 (Eįuivalvi, lateribus angulato-rotundatis, latere postico subcom- 

 presso-expanso ; albidd, corio corneo tenui cceruleo-viridescente, 

 epidermide fuscd indutd; radiatim costatd, costis sex vel septem et 

 triginta, planiusculis, umbones versus obsolete crenulatis ; umboni- 

 btis subapproximatis ; Ugamenti ared angustd, declivi. 



Conch, Icon. Arca, pL 9. f. 59. 



Hab. ? 



The nearest approach to the Arca inaguivalvis , but a shell of more 

 compressed growth ; the valves exhihit a still greater rlisparity of size, 

 and the ribs are rather more numercus. 



Arca crebricostata. Arca testd elongato-guadratd, ccguivalvi, la- 

 teribus superne angulatis, antico infra rotundato, postico elongato- 

 extenso, subattenuato ; albd, epidermide fuscd holosericd indutd ; 

 radiatim costatd, costis numerosissimis, tribus vel guatuor et qua- 

 draginta, planis, latiusculis, creberrimis, anterioribus sulco diiisis, 

 subtilissinie crenulatis; umbonibus latis, subapproximutis ; Uga- 

 menti a)-ed angustd, declivi. 

 Conch. Icon. Arca, pi. 9. f. 61. 



Hab. . ? 



The ribs of this species are very characteristic, being comparatively 

 broad, flat, very close-set, and more in nuraber than in any other of 

 tiic genus. 



