Shell small to large, inequivalve, with numerous irregular and fragile lamellae or scales on the right (upper) valve, and serrated foliations or crenulations on the left valve (lower one), which is attached to the hard substract. Hinge: no cardinal teeth but some marginal denticles (chomata) on both sides of the resilifer.
 

Shell small to medium-sized, with a rather deep lower valve, less lamellar than in most of the other members of the family, and hence more solid; « valves with the margins plaited, and clasping into each other. » – W. Swaison: The elements of modern conchology, London 1835, p.39. Species of this genus are « irregularly subcircular to elongate dorsoventrally, with thin shells cemented by a variable part of the left valve, often with clasper spines which are usually short and adnate throughout their length. Both valves may be convex, the left more so than the right, and generally plicate, although the right valve may have plications limited to a small region; the plications are small, rounded, closely spaced, with rounded grooves between. The shell margin is generally zigzag along some part, even when plications of the right valve are poorly developed. The external surfaces are without pustules, and usually smooth, often with a waxen texture, rarely with short lamellae at growth rests, which may form a few hyote spines on both valves. The exterior is often white, but usually blue or purple, sometimes rayed with darker stripes. The shell interior is often white, varying to light or dark green; the chomata are usually only ostreine, limited to the margin near the hinge, but occasionally supplemented by lophine chomata in the right valve also, extending to the ventral margin. » – H. W. Harry: “Synopsis of the supraspecific classification of living oysters”, The Veliger vol.28(2), Berkeley 1985, p.137.
 

Similar to the cupped oysters of the genus Crassostrea Sacco, but with some genetic differences (Salvi & Mariottini, 2016). Many scientists published, however, a common answer to this assessment, “The proposed dropping of the genus Crassostrea for all Pacific Cupped Oysters and its replacement by a new genus Magallana: a dissenting view”, Journal of Shellfish Research vol.36(3), in which they set out six reasons to withdraw this generic change.
 

Medium-sized to large shells, «  subcircular or subtriangular or rarely elongate dorsoventrally, the right valve nearly flat, with wide, overlapping, appressed, very thin and brittle lamellae, arising from rather closely spaced, evenly formed growth lines. The left valve is moderately inflated, with variable but usually distinct, small radial plicae, and a few growth rests which have only narrow, thick lamellae, or none at all. Chalk deposits in the inner shell layer are regularly present, in a distinctive pattern in older shells: a broad area behind and below the adductor muscle scar, with narrower extensions dorsad along both margins. The interior is usually white, occasionally washed with lavender or purple; similar color on the exterior of the shell is chiefly limited to young stages of growth. The chomata are very small, inconspicuous, ostreine, limited to the margin near the hinge, and sometimes disappearing in older specimens. No commissural shelf is defined in the shell. » – H. W. Harry: “Synopsis of the supraspecific classification of living oysters”, The Veliger vol.28(2), Berkeley 1985, p.142.
 

Shells medium-sized, « the attached part of the left valve of larger shells forming about half the total area of the valve or less, and the upturned part of the left valve with prominent, regular and continuous plications, often acutely crested, with broad, flat or concave troughs between. The right valve is flat, but projecting along the margin are short lobes, coordinate with the plications of the left valve; a few flattened plications, closely spaced, may develop on the right valve as the marginal lobes are extended. […] In protected situations the right valve of larger shells may be densely covered with imbricated lamellae, brown or purple, which have no radial striations (contrast Striostrea s.s.), but most right valves, exposed to the action of the waves, are eroded in a characteristic fashion. The abrasion forms broad, irregularly rounded pits, showing sharp or beveled contours, with the bottom of the pits often floored by a sheet of dark borwn, thick conchiolin (which becomes very brittle and fractures in dry shells). The hinge line is long, the left beak is often very large, and the left subumbonal cavity may often be as large as the rest of the shell’s interior; ostreine chomata are present, small but prominent, often widely spaced and extending to and across the ventral margin. Inside, the valves are essentially white, but a wide blue or purple margin may be present; the interior is porcellaneous; large brown patches of thick conchiolin are frequent, chiefly in the right valve. The attachments of the secondary mantle margin retractor muscles often leave a line of small, circular scars parallel to the front and hind margins of the shell. The valves are not brittle and flaky as in Striostrea. One or both adductor muscle scars may be darker than the general shell interior. » – H. W. Harry: “Synopsis of the supraspecific classification of living oysters”, The Veliger vol.28(2), Berkeley 1985, p.150.