ARC A 



Genus Am a, Linnaeus. 



Tata vel tequivalvis, vel inaquivalvis, inaquilateralis, ven- 

 tricosa, nunc radiatim cost at a, nunc simplex, epider- 

 midevelj \ indutd; umbonibus promi- 



nentibus, plus minusve distantibus, liganu 



o gradatim diseedentibus. Cm-do linearis, 

 ientibus minimis, numerosissimis. Ligamen- 

 i mum, ad aream inter umbon 

 ' -,iji'e bysso instructutn. 

 Shell equivalve or inequivalve, inequilateral, ventricose, 

 sometimes radiately ribbed, sometimes simple, co 

 vered with either a hairy or a bristlj 

 umbones prominent, more or less distant, receding 

 from each other according to the incres 

 ligamentary area. Hinge linear, strai 

 small and very numerous. Ligament external, at- 

 tached to the area between the umbones. Animal 

 often furnished with a byssus. 

 The Area present a very distinct and accurately de- 

 i 

 gards thi animal, according to the descriptions of Poli 

 and Des shell now und< I 



The cardinal portion of the shell of Area is of very pe- 

 culiar structure ; the umbones are, with I 



i I. -- vt idely separat d from oft< n to 



a very considerable extent, by the interposition of a 

 large area which sustains the ligament, deposited either 

 over its rutin' surface, or in superficial lozenge-shaped 

 grooves. The hinge is composed of a long rectilinear 

 series of tine plate-like teeth, varying from between 

 thirty and forty to one hundred and twenty or more in 

 number, in each valve ; the teeth of one valve interlock 

 closely with those of the opposite valve, and the hinge 

 or cardinal axis thus constituted runs through the centre 

 of the ligamentary area. 



The Arew are susceptible of being divided into two 

 very distinct groups or sections, and various modifica- 

 tions arise in their growth, composition, and sculpture, 

 from the difference of habit in their animal inhabitants. 

 The Area of the first division live free and unattached, 

 their shells are for the most part solid and ponderous, 

 and all are without exception disposed in strongly deve- 

 loped ribs radiating from the umbones, giving a crenu- 

 lated margin to the valves, by which they interlock one 

 with the other. Another interesting feature in this 

 group is that the shell is often inequivalve, the margin 

 of the left valve extending beyond that of the right, and 

 generally exhibiting a more elaborate style of sculpture, 

 as if the left mantle of its animal inhabitant were more 

 highly organized than the right. 



The Areas of the second division live attached to 



stones, &c, by a strong muscular byssus which is cx- 

 serted though an opening in the margin of each valve; 

 their shells are of much lighter structure and composi- 

 tion, and being seldom distinctly ribbed, the margin of 

 the valves is not crenulated. The protection moreover 

 which thi iring Arcce gain by their strong 



power of attachment allows of a much more simple con 

 dition of the hinge ; the teeth are consequently smaller, 

 and the ligament is weaker. 



Yew of the Area have ever been figured, though seven- 

 and-twi n added ten years since by Mr. 



Sowerby to those already published by Lamarck. Say, 

 and others ; these therefore, together with a considerable 

 number of new ones since acquired, constitute a very 



interest!; 



Species 1 . (Mus. Cuming.) 

 Arca \rx. Area testa obliqui rhomboided, turgidd, in- 

 eequivalvi, radiai < \stis duabus vel tribus 



angustis, nodulosis, valve dextralis costis 

 pie^/iiis sublcevibus ,- umlwniliii.i /iraiuine/iti/ii/s, distan- 

 tibus, fix obliquis ; a/bidtl, epiilermiile fused, earned, 

 teiini, ad anticaia sulearum partem setii/erd, indiitii. 



i Ark. Shell obliquely rhomboid, swollen, in- 

 equivalve, radiately ribbed, ribs two or three and 

 twenty in number, narrow, nodulous, posterior ribs 

 of the right valve nearly smooth ; umbones promi- 

 nent, distant, a little oblique; white, covered with 

 a thin, brown, horny epidermis, bristly on the an- 

 terii i ioves. 



Sowerby, Pro. Zool. So,-., 1833. 



//<(/;. Xi] ixapi, South America (found in sandy mud at 

 the depth of twelve fathoms) ; Cuming. 

 A small oblique gibbous species. 



Fig. 2. (Mus. Cuming.) 

 Arca gransis. Testa junior. 

 For description see Species IV. 



Area quadrilatera, Sowerby (not of Lamarck). 



After minutely comparing this shell, the identical 

 specimen described by Sc 

 of the Arca grandis, I ar 

 the young of that species. 



verby, with several examples 

 convinced that it is merely 



Species 3. (Mus. Cumiug.) 

 a obesa. Arca testa ovatd, incequivalvi, tenuiculd, 

 ventricosa, latere antico rotundato, postico subangu- 

 lato, extenso ; albd, epidermide fused, inter castas 



December 1843. 



