A R C A. 



Plate IV. 



Species 20. (Mus. Cuming.) 

 bbosa. Area testd rotundato-ovata, gibbosissimd, 

 tequivalvi, lateribus subangulatis, infra rotundatis, 

 latere antico breviore ; albidd, epidermide fused in- 

 dutd; radiatim costatd, costis sex vel septem et vi- 

 ginti, rottiitt/it/is, angustis, leviter nodulosis ; liga- 

 menti ared mediocri. 

 The gibbous Auk. Shell rotundately oval, very gib- 

 bous, equivalve, sides ratber angulated, rounded 

 beneath, anterior side shorter; white, covered with 

 a brown epidermis ; radiately ribbed, ribs six or 

 seven and twenty in number, round, narrow . slightlj 

 nodulous ; ligamentary area middling. 

 Reeve, Pro. Zool. Soc, 1844. 



Hah. ? 



This shell is almost as round as the Arcapihla, from 

 which it differs however essentially in having the urn- 

 bones obliquely turned to the anterior, and the ribs 

 rounded. 



Species 21, (Mus. Cuming.) 



Ahca Americana. Area testa ovato-oblongd, ■ 



cequivalvi, lateribus rotundatis, antico brevissimo, 

 coarctato ; alba, epidermklr crassiuscule 

 fused, inter eostas setosd, obtectd ; radiatim costatd, 

 costis ml quinque et triginta, planulatis, leevibvs, con- 

 fcrtis, interstitiis jinfunde ineisis ; ligament i and 

 angustissimd ; umbonibus seferl amino tangentibus. 



The American Ark. Shell ovately oblong, very ine- 

 quivalve, sides rounded, the anterior very short and 

 contracted ; white, covered over with a rather thick 

 blackish brown epidermis, which is bristly in the 

 interstices between the ribs; radiately ribbed, ribs 

 about five-and-thirty in number, flat, smooth, close- 

 set, interstices deeply cut ; area of the ligament 

 very narrow ; umbones almost touching each other. 



Gray, Wood, Ind. Test. Supp., PL II. fig-. 1. 



Huh. South Carolina, United States. 



This interesting species might easily be confounded 

 with the Area pexata, which is found in the same lo- 

 cality. The ribs of the Area Americana are six and 

 thirty in number, very close-set, and perfectly flat and 

 smooth ; whilst those of the Area pexata are only thirty 

 in number, divided by a fine groove, and the interstices 

 between them are delicately cancellated. 



Species 22. (Mus. t't 



Aki \ pexata. Area lestii suborbiculari-ovatd, 



aquivalvi, lateribus rotundatis, antico breviore, leviter 

 coarctato; albd, epidermide crassiusculd, nigricante- 



fitSCd, inter eostas setosd, obtectd ; radiatim eostata. 



costis tricenis, audio suleo exili divisis, interstitiis 



ancellatis ; liijamenti ared iini/ustissinid ; itni 

 lionilitts se l'i re mutito tangentibus. 

 The high-napped Ark. Shell somewhat orbicularly 

 ovate, scarcely inequivalve, sides rounded, anterior 

 shorter ; white, covered over with a rather thick 

 blackish brown epidermis, which is bristly in the 

 interstices between the ribs; radiately ribbed, ribs 



thirty in number, divided in the middle by a fine 

 into rstici - delicately cancellated ; ligamen- 

 t.n\ ar.a very narrow ; umbones almost touching 

 each other. 

 Sat, Journ. ^cad. nat. sci. Phil., vol. ii. 



Huh. South Carolina, Tinted States. 



Besides the differences already noted between this and 

 the preceding species, it may be observed that the Area 

 i a smaller and rounder shell than the h a 

 oviformis, and less inequivalve. 



Species 23. (Mus. Cuming.) 

 Arca multicostata. Area testa quadrato-rhombed 



Solidd, iri/niea/ri, Interilins stipend atleniiatis, unijit- 

 lalis. antico breviore, infra ventriioso-roluiidato, pus- 

 tico aihjiilato-ileeHei ; churned, epidermide fused cor- 

 ned, inter eostas Sttbvelutind, obtectd ; radiatim cos- 

 tula, costis quinque vel see et triginta, subangustis, 

 rotundatis, lavihus, untieis leviter granulosis ; liga- 

 incnli urea iuliiiseuld ; umbonibus subujipro.rimatis. 

 The many-ribbed Auk. Shell squarely rhomboid, so- 

 lid, equivalve ; sides attenuated and angulated at 

 the upper part, anterior side shorter, ventricosely 

 rounded beneath, posterior angularly extended 

 downwards; ivory-white, covered over with a 

 brown horny epidermis, which is a little velvety 

 between the ribs ; radiately ribbed, ribs about six- 

 and-thirty in number, rather narrow, rounded, 

 smooth, anterior ribs slightly granulous ; area of 

 the ligament rather broad ; umbones somewhat ap- 

 proximated. 



Janu iry 1S44. 



