ARCA.— Plate XI. 



wards the upper part, with a blunt keel running 

 from the umbone to the margin, ventral margin 

 more or less gaping ; brown, fleshy white about 

 the middle, painted with waved streaks of brown ; 

 radiately ribbed, ribs rude, irregular, scaly- wrinkled, 

 anterior middle ribs nearly obsolete ; area of the 

 ligament flatly concave. 



Linn-eus, Syst. Nat. (Gmelin's edit.), p. 3306, No. 2. 



Hab. Mediterranean (attached to stones). 



It is scarcely necessary to observe how greatly this 



species varies in growth according to its circumstances 



of attachment. 



Species 73. (Mus. Metcalfe.) 

 Arca imbricata. Area testd abbreviato-oblonga, latere 

 antico gibboso, plus minusve tumido,postieo attemtato, 

 truneatu, carina obtusd ab umbone ad marginem de- 

 currente, margine ventrali bysso h'tante ; nigricante- 

 fuscd, antie'e cinereo-caruleo pallide tinctd; parte 

 mediand antiedque subtilissime radiatim liratis, liris 

 striis elevatis longitudinalibus imbricato-decussatis, 

 parte posticd costatd, costis obtusis, subdistantibus ; 

 umbonibus inclimtto-apprnximatis ; ligamenti area 

 vald'e concavd, rhombo sidcifero parvo, antice depo- 

 sit o. 

 The imbricated Ark. Shell shortly oblong, anterior 

 side gibbous, more or less swollen, posterior at- 

 tenuated, truncated, with a blunt keel running from 

 the umbone to the margin, ventral margin mode- 

 rately gaping; blackish brown, palely tinged ante- 

 riorly with ashy blue ; middle and anterior portions 

 of the shell very finely radiately ridged, ridges im- 

 bricately crossed with raised longitudinal stria;, 

 posterior portion ribbed, ribs blunt, rather distant ; 

 umbones bent over, rather approximated ; area of 

 the ligament very concave, diamond-shaped space 

 bearing the hgamentary grooves small, situated at 

 the anterior end. 

 Bruguiere (not of Poli), Enc. Meth. (1789) vers, p. 98. 

 Arcaumb<matal\ Lamarck 

 Arca ventricosa ? J 

 Hab. East Indies (found under stones). 



This species was described with great minuteness by 

 Bruguiere in the ' Encyclopedic Methodique,' about two 

 years before the publication of Poli's beautiful work, in 

 which the same specific title is employed in reference to 

 another of the same genus. 



Species 74. (Mus. Cuming.) 

 Arca truncata. Arca testii clongato-obiongd, latere 

 antico brevissimo, postico elongalo, truncate,, carina 

 obtusd ab vmbone ad marginem decurrente, margine 

 ventrali bysso hiante ■ cinereo -fused, strigis fuscis 

 ttiidat is umbones versus pallide tinctd; parte mediand 

 antiedque subtilissime radiatim liratd, liris striis ele- 

 vatis longitudinalibus imbricato-decussatis, parte pos- 

 ticd costatd, costis obtusis, subdistantibus, interstitiis 

 rugosis ; umbonibus mediocribus ; ligamenti aredpla- 



niusculo-concavd,fusco-strigatd ; sulcis Ugamentariis 

 angustis, perpaucis, in area parte anticd tantum in- 

 cisis. 



The truncated Ark. Shell elongately oblong, anterior 

 side very short, posterior elongated, truncated, with 

 a blunt keel running from the umbone to the mar- 

 gin, ventral margin gaping ; ashy brown, palely 

 stained towards the umbones with brown waved 

 streaks ; middle and anterior portions of the shell 

 finely radiately ridged, imbricately crossed with 

 raised longitudinal stria?, posterior portion ribbed, 

 ribs obtuse, rather distant, interstices wrinkled ; 

 umbones middling ; area of the ligament rather 

 flatly concave, streaked with brown ; ligamentary 

 grooves narrow, few in number, engraved on the 

 anterior part of the area only. 

 Byssoarca truncata, Sowerby, Pro. Zool. Soc, 1833. 



Hab. Gallapagos Islands, Pacific Ocean (found attached 

 to stones) ; Cuming. 



An interesting species, partaking of the characters of 

 Arca zebra and imbricata, having the anterior ligamen- 

 tary grooves of the former with indications of the same 

 zebra-like painting, and the imbricated sculpture of the 

 latter with the same truncated extremity. 



Species 75. (Mus. Cuming.) 



Arca Pacifica. Arca testa trupezio-oblongd, latere an- 

 tico brevissimo, parvo, superne acuminata, postico 

 maximo, alato-expanso, emarginato, carind obtusd ab 

 umbone ad marginem decurrente, margine ventrali 

 bysso comparat'c paululum hiante; albidd, rubido 

 tinctd, strigis rubidis latis undique pictd ; radiatim 

 costatd, costis irregularibus, nodosis, inediis mino- 

 ribus ; umbonibus remotissimis ,• ligamenti ared latis- 

 simd, plano-concavd, rhombo sulcifero, maximo, corio 

 corneo nigricante-fusco induto, sulcis Ugamentariis 

 latis, subdistantibus. 



The Pacific Ark. Shell trapezoidly oblong, anterior 

 side very short, small, acuminated at the upper part, 

 posterior very large, expanded like a wing, emar- 

 ginated, with a blunt keel running from the um- 

 bone to the margin, ventral margin gaping compa- 

 ratively but little ; whitish, tinged with red (brick - 

 red), everywhere painted with broad waved streaks 

 of the same colour ; radiately ribbed, ribs irregular, 

 knobbed, middle ribs smaller ; umbones very re- 

 mote from each other ; area of the ligament very 

 broad, flatly concave, diamond-shaped space enclo- 

 sing the ligamentary grooves very large, covered 

 with a dark brown horny cuticle, grooves broad and 

 rather distant. 

 Byssoarca Pacifica, Sowerby, Pro. Zool. Soc, 1833. 



Hab. St. Elena, South America (found adhering to each 

 other in bunches attached to rocky ground at the 

 depth of from six to eighteen fathoms) ; Cuming. 

 This beautiful species may be recognized by its strong 



ribbing, and by the wing-like expansion of the posterior 



side. 



