IOO PATAGONIAN EXPEDITIONS I PAL/EONTOLOGY. 



1897 (Dec.) O. percrassa v. Ihering (non Conrad), in: Rev. Mus. Paul., 



v. 2, p. 221, pi. 9, f. 53, textfig. i. 

 1897 O. 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. hatcheriv. 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 : (i) 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- 



