ARC A. 



Plate XIII. 



Species 83. (Mus. Cuming.) 

 Arca barbata. Area testd oblongo- vel elongato-ovatd, 

 subcompressd, lateribus rotundatis, latere antico valde 

 breviore, margine ventrali bysso paululum hiante ; 

 fuscescente, rubido-fusco plus minusve tinctd et varie- 

 gatd, epidermide setosd indutd ; radial im striata, 

 striis elevatis, granulosis, subirregularibus ; umbo- 

 nibus compresso-approximatis ; ligament i area an- 

 gustd, profundi- declivi. 

 The bearded Ark. Shell oblong or elongately ovate, 

 a little compressed, sides rounded, anterior side 

 much the shorter, ventral margin a little gaping ; 

 light brown, more or less variegated and stained 

 with dark reddish brown, covered with a bristly epi- 

 dermis ; radiately striated, striae raised, granulous, 

 rather irregular; umbones compressly approximated; 

 area of the ligament narrow, deeply bent inwards. 

 Linnaeus, Syst. Nat. (Gmelin's edit.), p. 330G. 

 Hab. Mediterranean and other seas of Europe. 



This ancient and well-known species varies consider- 

 ably in shape according to its situations of attachment ; 

 it differs from the preceding species in not having the 

 white umbonal rays. 



Species 84. (Mus. Cuming.) 

 Arca angulata. Arca testa rhomboided, plano.com- 

 pressd, lateribus acutangulis, antico acuminato, pos- 

 tico ared latiusculd formante, ared superne serratd, 

 costatd, costis nigris, crenulatis, margine ventrali 

 bysso latissime hiante ; fused, epidermide cristatd ad 

 extremitatem posticam indutd; umbonibus parvis, 

 remotissimis, plerumque decorticutis ,■ ligament i area 

 maxima, plana, umbones versus inclinatd, lineis fuscis 

 obliquis paralle/is undique pictd ; rhombo ligamen- 

 tario grandi, corio fusco induto, antice adjecto, sulcis 

 nunc plurimis, mine perpaucis. 

 The angulated Ark. Shell diamond-shaped, flatly 

 compressed, sides sharply angled, anterior side 

 acuminated, posterior forming a rather wide area, 

 area serrated along the upper edge, ribbed, ribs 

 black, crenulated, ventral margin very widely 

 gaping for the passage of the byssus ; brown, co- 

 vered at the posterior end with a thick laminated 

 tuft of epidermis ; umbones small, very remote, 

 generally decorticated ; area of the ligament very 

 wide, flat, inclined towards the umbones, every- 

 where painted with oblique brown parallel lines ; 

 ligamentary space large, covered with' a brown cu- 

 ticle, situated on the anterior portion of the area, 

 grooves sometimes many in number, sometimes 

 very few. 

 King, Zool. Journal, vol. v. p. 336. 



Hab. Cumberland Bay, Juan Fernandez (found attached 



to a branch of coral at the depth of eighty fathoms) ; 



King. 



The ligamentary area of this extraordinary species is 



composed of two inclined planes gently descending on 



each side from their line of union in the centre, and it 



constitutes at least two-thirds of the entire shell, which. 



(as Captain King significantly expresses it) has very 



much the appearance of a flat Indian canoe. 



Species 85. (Mus. Cuming.) 

 Arca mutabilis. Arca testd oblongd, lateribus inferne 

 angulatis, antico breviore, infra rotundato, postico 

 longiusculo, angulato, carind ah umbone ad marginem 

 decurrente, margine ventrali bysso hiante; pallid'e 

 fused, epidermide lamellatd prope marginem indutd ; 

 radiatim striatd, striis elevatis, fori iter granulosis, 

 ared posticd nigricostatd ; umbonibus subapproxi- 

 matis ; ligament i ared latiusculd, concavd, sulcis liga- 

 mentariis perpaucis, anticis. 

 The changeable Ark. Shell oblong, sides angulated 

 at the lower part, anterior side the shorter, rounded 

 beneath, posterior rather long, angulated, with a 

 keel running down from the umbone to the margin, 

 ventral margin gaping for the passage of the byssus ; 

 pale brown, covered near the margin with a lamel- 

 lated epidermis ; radiately striated, striae raised, 

 strongly granulated, posterior area ornamented with 

 black ribs ; umbones somewhat approximated ; 

 area of the ligament rather broad, concave, liga- 

 mentary grooves very few in number, anterior. 

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

 Hab. Isle of Plata, West Columbia (found under stones) : 

 Cuming. 

 The dark ribs on the posterior area of this shell con- 

 stitute one of its most prominent specific features. 



Species 86. (Mus. Stainforth.) 

 a tortuosa. Area testd elongato-tortuosd, paral- 

 lelopipedd formante, ina>quivalvi, latere antico su- 

 perne acuminato, infra rotundato, postico angulato, 

 latissime alato-e.rpanso, oblique carinato, carind ser- 

 ratii, ab umbone ad marginem decurrente ; alba, car- 

 neo-fuscescente pallide tinctd ; radiatim striatd, striis 

 elevatis, area valva sinistralis subobsolelis ; umboni- 

 bus minut is, approximatis ; ligamenti area longissimd, 

 ungustissimti, declivi. 



; twisted Ark. Shell elongately twisted, forming 

 a parallelopiped, inequivalve, anterior side acumi- 

 nated at the upper part, rounded beneath, posterior 

 angulated, very widely wing-likeexpanded, obliquely 



April 1844. 



