ARC A. 



Species 63. (Mus. Cuming.) 

 Arca Aviculoides. Area testd elongato-oblongd, vix 

 aquivalvi, lateribus superrie angulatis, auriculatis, 

 postico pracipu'e, infra rotundutis, latere antico bre- 

 vissimo, postico elongato ; alba, fuscescente postice 

 virgatd, epidermide fused, inter castas setosd, indutd; 

 radiatim costatd, costis septem vet oeto et triginta, 

 planulatis, confertis, anticis subtilissiiui erenulatis ,- 

 ligament! area angustd. 

 The Avicula-like Ark. Shell elongately oblong, 

 scarcely equivalve, sides angulated at the upper 

 part, auriculated, the posterior side especially, 

 rounded beneath, anterior side very short, posterior 

 elongated ; white, faintly striped on the posterior 

 side with brown, covered with a soft brown epider- 

 mis that is bristly between the ribs ; radiately rib- 

 bed, ribs seven or eight and thirty in number, flat, 

 close-set, anterior ribs very finely crenulated ; area 

 of the ligament narrow. 



Arca uuriculata, Sowerby, Pro. Zool. Soc, 1S33. 

 Hub. St. Elena, South America (found in a muddy 

 bottom at the depth of ten fathoms) ; Cuming. 

 I am obliged to publish this pretty little species under 

 a new name, in consequence of the word uuriculata 

 having been pre-occupied by Lamarck (vide Species 35. 

 Plate VI.). 



Species 64. (Mus. Cuming.) 



Arca occlusa. Arca testa subquadrata, vutd'e ijibbasd, 

 ineequivalvi, lateribus superne attenuato-ungulatis ; 

 alba ; radiatim costatd, costis septem vel oeto et vi- 

 ginti, subdistantibus, valva dcvtralis Icevibus, sinistra- 

 lis iiuduliisis ; umbunibus prominentibus, peentiariter 

 approximate ; ligamenti area subdeclivi. 



The closed Ark. Shell nearly square, very gibbous, 

 inequivalve, sides attenuately angled at the upper 

 part ; white, radiately ribbed, ribs seven or eight 

 and twenty in number, rather distant, those of the 

 right valve smooth, of the left valve nodulous ; um- 

 bones prominent, peculiarly approximated ; area of 

 the ligament rather bent inwards. 



Reeve, Pro. Zool. Soc, 1S44. 



Hab. ? 



The umbones of this shell 



i closely approximated 



over the ligamentary area, as to prevent the valves 

 from opening beyond the extent of about a quarter of 

 an inch. 



Species 6.3. (Mus. Metcalfe.) 



Arca ambigua. Arca testa subquadratd, tenuiculd. in- 

 mquivalvi, lateribus subatteiiuato-angulatis, antico in- 

 fra rotundato, postico angu/ato ; albidd, epidermide 

 fused, inter cos/as squamosa, indutd ; radiatim cos- 

 tatd. costis tribus vel quatuor et triginta, angustis, 

 rotundutis, anticis subti/issime rutundatis ; ligamenti 

 area subangustd. 



The ambiguous Ark. Shell somewhat square, rather 

 thin, inequivalve, sides rather attenuately angulated. 

 anterior rounded beneath, posterior angulated ; 

 whitish, covered with a brown epidermis, scaly be- 

 tween the ribs ; radiately ribbed, ribs three or four 

 and thirty in number, narrow, rounded, the ante- 

 rior ribs very finely nodulous ; area of the ligament 

 rather narrow. 



Reeve, Pro. Zool. Soc, 1S44. 



Hub. ? 



The Arca cistula is the nearest allied species to this. 



Species 66. (Mus. Cuming.) 

 Arca cepoides. Area testd subquadrato-ovatd, tenui, 

 ventricosd, in&quivalvi, lateribus angulato-rotundatis ; 

 albidd, fuscescente pallid e tinctd, corio corneo ceeru- 

 lescente, epidermide fused, inter castas squamosa, in- 

 dutd ; radiatim costatd, COStis duabus et triginta, Ice- 

 vibus, planulatis, costis valval sinist rails sublatioribus ; 

 umbonibus tumidis ; ligamenti area latiusculd, valdi 

 declivi. 

 The onion-like Ark. Shell somewhat squarely ovate, 

 thin, ventricose, inequivalve, sides angularly round- 

 ed ; white, palely stained with light brown, covered 

 with a bluish horny cuticle, and a brown epidermis 

 that is scaly between the ribs; radiately ribbed, 

 ribs two and thirty in number, smooth, flat, those 

 of the left valve rather wider than those of the 

 right ; umbones swollen ; area of the ligament 

 rather wide, very much bent inwards. 

 Reeve, Pro. Zool. Soc, 1844. 



