47 



Arca hians. Arca testd eloncjato-cvatd. a-rjuhcdin a,,lki hlunte, 

 lateribus rotundatis. postico anenucUo-crlenso ; M.da,f>^ccsce,^ 



radiatim costatd. costis duabus vel tribus et trigmla, laUusculu, 



umbonibus subapproxmtttis ; ligamenti area angus:a, pofunde de- 



clivi. 

 Conch. Icon. Arca, pi. 9. f. 62. 



"t^Joi the Arca hians approaches some^hat to that of the 

 Arcacymblforrr>.is, but the species d.ffer most essentmily from each 

 other on eimination. In the Arca l^ans the valves -re eqaal he 

 anterior ribs are divided by an unusually broad groove, and the .heli 

 Spe at the anterior end'to the extent of about three-s.xteenths of 

 S^inch, a peculiarity of v'hich I have not obsen^ed the shghtest m- 

 dication in any other species of this divisiou of the Arca^. 



AucA occLUSA. Arca testd subguadratd, valde gibbcsd inceųmvalvK 



TteribuTsuperne attenuato-angulatis ; albd; rad^aUm cosatd. 



costs evtei vel acto et riginti, subdistantibus, vaivos de.trahs 



ZTbus%stralis nodulosis ■ umbonibus prominenttbus. pecuhar.- 



ter approrimatis ; ligamenti ared subdechvi. 



Conch. Icon. Arca, pi. 10. f. 64. 



?he^L of this Shell are so closely approximated over the 

 ligamentrry area. as to prevent the valves from opemng beyond the 

 extent of about a ąuarter of an mch. 



Arca ambigua. Arca testd subguadratd, tenuiculd inagnivalci, la- 

 te"Z subattenuato-angulatis, antico ivfra rotundato, P-^-° J-. 

 gulato; albidd, epidermide fuscd, inter costas sgvamosa , mduta 

 faZm costatd costis tribns vel guatuor et trųrnta, angustis 

 rotundatis, anticis subtilissime rotundatis; hgamentt arca suban- 



gusta. 

 Conch. Icon. Arca, pi. 10. f. 65. 



The Arca čistula is the nearest allied species to this. 

 Arca cepoides. Arca testd subquadrato-ovatd, tenui, verdricosd, 

 ^^^vi, lateribus angulato-rotu.datis ; ^idajuscesa^e^ 

 Udi tinctd, corio corneo carulescente, epidcnmde Jvsca, nte> 

 Tostas sauamosd,indutd; radiatim costatd, cost, s dnį^yU'^;- 

 ninta, llvibus, planulatis, costis valvce stmMrahssublat,oribus; 

 Lbonibus tumidis; ligamenti ared latiuscula, valde dechvi. 



^«f San Migtr'sttl'ALdca (found in sandy .nd) ; Cunung. 

 This is a fine^bold .pecies, but it does not exlubit any very str.king 

 peculiarity of character. 



A-^ca HANKEY^NA. Arcū testd ohHque ovatd, crassiusculd. valde 

 itbbosd, Uanidd, cegulvalvi, lateribus rotundatis, .uperne attenuatts 

 Vre aalico brevssimo, postico obUque e.ienso ; albu. eptdenmde 



