IOO 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
1897 (Dec.) O. percrassa v. Ihering (non Conrad), in: Rev. Mus. Paul., 
v. 2, p. 221, pi. 9, f. 53, textfig. 1. 
! ^97 D. patagonica v. Ihering (non d’Orbigny), ibid., p. 222, pi. 9, f. 2 
(O. orbignyi in tab.) (non O. patagonica, ibid., p. 326). 
1899 O. hatcheri v. Ihering, in: N. Jahrb. Min. Geol. Pal., v. 2, p. 8. 
1900 O. ingens Ortmann, in: Amer. Journ. Sci., v. 10, p. 379. 
Shell very variable in shape, circular, ovate, triangular, or elongated, 
becoming very large and thick with age. Lower (left) valve more or less 
convex, concave on inner side. Outer surface with concentric lines of 
growth, forming lamellae, which are sometimes very far apart from each 
other, giving thus a graduated appearance, but in other cases the lamellae 
are much crowded. Radial plaits in many cases completely absent, in 
others slightly and irregularly developed, but always less than in O. tor- 
resi. Upper (right) valve flat or slightly concave on the inner side, ex- 
ternally with lines of growth, forming lamellae, very rarely with traces of 
radial plaits. Beak of lower valve longer or shorter, often very much 
elongated, sometimes incurved or distorted. Area triangular, generally 
longer than broad, often very long, rarely broader than long. Ligament- 
groove deeper or shallower, broader or narrower. Beak and area of 
upper valve shorter than in the lower valve. Muscular impression large, 
situated generally a little below the middle of the shell and a little pos- 
teriorly (laterally). Margins of upper valve, close to the area, sometimes 
for a short distance with small crenulations (wrinkles or nodes) corre- 
sponding to small grooves in the other valve, but in most cases no traces 
of these crenulations are present. 
Largest specimen: Length, 255 mm; width, 162 mm. 
Remarks: The chief characters of this species are: 
1. The large size, and extremely thick shell. 
2. The situation of the muscular scar. 
3. The smooth margin of the inner side of the valves. 
4. The slight development of the radial folds. 
But even these characters are variable in a certain degree. Greater 
variations are shown in : (1) the outline, (2) the shape of area and beak, (3) 
the convexity of the valves, (4) size, and development of the surface-sculp- 
ture, especially of the lines of growth. 
Since there is an almost unparalleled confusion as to the identification 
and synonymy of the large .oyster of the Patagonian beds, it seems neces- 
