ARC A. 



Species 90. (Mus. Cuming.) 



Arca Helblixgii. Arca testa ovatd, p/ns minusve com- 

 pressd, latcribus rotundatis, postico superne obliqul 

 anyalato, antieo obtuse angulato, margine ventrali 

 bysso subirregulariter hiante ; albd, epidermide squa- 

 mosa versus marginem indutd ; radiatim striata, 

 striis elevatis, granulosis, alternis minor ibus, striis I 

 posticis dup/ieatis ; umboiiibus parvis ; ligamenti area 

 subangustd, profundi declivi. 



Helblixg's Ark. Shell ovate, more or less flatly com- 

 pressed, sides rounded, posterior obliquely angu- 

 lated at the upper part, anterior obtusely angulated, 

 ventral margin rather irregularly gaping for the I 

 passage of the byssus ; white, covered towards the 

 margin with a scaly epidermis ; radiately striated, 

 stria; raised, granulated, alternate striae smaller, 

 posterior striae double ; umbones small ; area of the 

 ligament rather narrow, deeply bent inwards. 



Bruguiere, Enc. Meth. vers, vol. i. p. 99. 

 Arca Candida Helblingii, Chemnitz. 



Halt. St. Elena and Monte Christi, West Columbia, and 

 island of Corrigidor, Philippines (found under | 

 stones at low water) ; Cuming. 

 This species was dedicated by Chemnitz to Professor 



Helbling, a celebrated naturalist of the last century 



living at Mantua. 



A delicate light brown shell, beautifully striated, with 

 soft scattered epidermis. 



Species 91. (Mus. Cuming.) 



Arca texella. Arca testd subcylindraceo-oblongd, Mo- 

 diolaformi, tenui, subpellucidd, lateribus obtuso-ro- 

 tundatis, margine ventrali vix hiante ; pallide fits- 

 cescente, epidermide molli leviter indutd; radiatim 

 siibtilissim'e striatii, striis granulosis ; umboiiibus ob- 

 tusis, antice incurvis ; ligamenti area antice latiusculd, 

 postie'e laneeolato-aeuminatd. 



The delicate Ark. Shell somewhat cylindrically ob- 

 long, Modiola-shaped, thin, rather transparent, 

 sides obtusely rounded, ventral margin very slightly 

 gaping for the passage of the byssus ; pale light 

 brown, slightly covered with a soft epidermis ; very 

 finely radiately striated, striae granulous ; umbones 

 obtuse, incurved anteriorly ; area of the ligament 

 rather broad at the anterior end, lanceolately acu- 

 minated posteriorly. 



Reeve, Pro. Zool. Soc, 1844. 



Hub. Island of Burias, Philippines (found under stones 

 at low water) ; Cuming. 



Species 92. (Mus. Cuming.) 

 Arca gradata. Arca testd oblongd, latere antieo rotun- 

 dato, postico angulato, margine ventrali vix hiante . 

 albii/d, epidermide nulla : radiatim costata, costis an- 

 gustis, numerosis, creberrimis, fortibus, noduloso- 

 granu/atis, granulis prominentibus, gradibus concen- 

 tricis descendentibiis formantibus, costis posticis ver- 

 sus marginem duplicatis ; umboiiibus subapproximatis; 

 ligamenti area antice latiusculd, postie'e laneeolato- 

 aeuminatd. 

 Thestep-builtArk. Shell oblong, anterior side rounded, 

 posterior angulated, ventral margin scarcely gaping; 

 whitish, no epidermis ; radiately ribbed, ribs narrow, 

 numerous, close-set, strong, nodulously granulated, 

 granules prominent, forming descending concentric 

 steps, posterior ribs double towards the margin ; 

 umbones somewhat approximated ; area of the 

 ligament rather wide at the anterior end, lanceo- 

 lately acuminated at the posterior. 

 Broderip axd Sowerby, Zool. Journal, vol. iv. p. 365. 

 Hub. St. Elena, West Columbia (found attached to 

 stones) ; Cuming. 

 The sculpture of this shell stands out in such promi- 

 nent relief, that it has the appearance of Chinese carving. 



Species 93. (Mus. Cuming.) 

 Arca lacerata. Arca testa subtjuadrato-ob/ongd, plant- 

 usculo-compressd, antice breri, postie'e alato-expansd, 

 lateribus obtuso-rotunda/is, margine ventrali bysso vijr 

 hiante; rubido -fused ; epidermide fused squamosd, 

 setariimqiie seriebus radiantibus, posterioribus forti. 

 oribus, indutd; radiatim striatd, striis leviter elevatis, 

 granulosis ; umboiiibus subappro.iimatis, antice ad- 

 jectis ; ligamenti ared angustd, declivi. 

 The ragged Ark. Shell somewhat squarely oblong, 

 rather flatly compressed, short anteriorly, wing- 

 like expanded posteriorly, sides obtusely rounded, 

 ventral margin but slightly gaping for the passage 

 of the byssus ; reddish brown ; covered with a scaly 

 epidermis and a number of radiating rows of 

 bristles, the posterior of which are the strongest ; 

 radiately striated, striae slightly raised, granulated ; 



April 1844. 



