muculiba;. 45 



ami rather prominent below. Veutral margin rather finely crenated 

 within, arched, rising posteriorly. Beaks prominent. Lunular 

 area shining, rather obscurely, yet impressly defined, somewhat 

 pouting, concentrically wrinkled, without radiating lines or granules. 

 Opposite dorsal area smoolhish or concentrically wrinkled. About 

 twenty teeth on one side, and ten on the other. — The northern and 

 southern forms differ to some considerable extent; the surface in 

 the former is comparatively smooth, and its epidermis is yellowish ; 

 in the latter (or at least in the Mediterranean examples*) the 

 epidermis is of a dusky-olive, the sculpture is much more decided, 

 and the radiating lines often pervade the central portion likewise. 

 Bruguiere's description of his A. maryaritacea (Hist. Nat. Vers, 

 109), for which Area nucleufs of Linnfeus is indicated as a probable 

 synonym, is rather generic than specific; he admitted but a single 

 species of nacreous Area, which he ascribed to St. Domingo, but 

 referred likewise to England as a habitat, on the authority of Da 

 Costa. As Petiver's rude figure (Gaz. pi. 17, f. 9) of the present 

 species is cited by him as the most characteristic, we may fairly 

 regard his margarifacea as, in the main, identical with the com- 

 monest of our known Nueidce. 



3. N. STEiOLATA, Adams, f. 129. — T. sp. prsecedeuti simillima, 

 sed semper pallide olivacea, magis obliqua, compressa, sublsevigata 

 (oculo autem armato undique striolis vix elevatis dense radiata) ; 

 rugje coucentricae obsoletas et iequaliter difl'usas. Lunula baud 

 defiuita. — Closely resembling tuicleiis, but more oblique, com- 

 pressed, always of a pale-olive, smooth to the eye, but under a 

 powerful lens the general surface everywhere densely radiated 

 with scarcely raised striolse : the concentric wrinkles obsolete and 

 evenly diffused. Lunular area undefined. — The likeness to a young 

 Mediterranean nucleus is most striking. Owing to the thinness 

 of the epidermis there is an external iridescence around the lunu- 

 lar area. 



4. N. EADIATA, llaidey, f. 123, 124. — T. N. nucleo simillima, 

 sed major, longior, magis obliqua magisque inaequilateralis, radiis 

 aliquando rubro-aurautiis nonnunquam fumosis picta, magis 

 minusve compressa. Cutis nitidiuscula, olivacea, in junioribus 

 olivaceo-lutea. — With the shape, sculpture, and general characters 

 of nucleus, but larger, longer, more oblique, still more inequilateral 



* I suspect that the iV. rtigulosa of Sowerby (Coneh. Illust. N"ucula, n. 28, f. 19, 

 copied in Haul. Rcc. Bivalves, p. 1 71, pi. 20, f. 10) was constituted from this variety ; 

 at least, I know no species which so greatly resembles it. It is thus described : — 

 " Ovate-trianeular. with a greenish epidermis, covered with rugiJose stria;; angles 

 obtuse ; anterior \i.''. posterior) slope with a central clrvatiun ; posterior (/.c. anterior) 

 slope somewhat rounded ; ventral maigiu very lincly ereuulated, 0'5." 



