A. K Ortmann — Oysters of Patagonia. 355 



Akt. XXXVIII. — On some of the large Oysters of Patagonia ; 

 by Dr. Arnold E. Ortmann. With Plate XI. 



Since there is much confusion in regard to the identifica- 

 tion of the giant oysters of Patagonia, it seems well to describe 

 briefly those forms collected by Mr. Hatcher, and to give their 

 proper geological relations. 



1. Ostrea hatclieri nov. spec. (Plate XI, fig. 1.) 



Shell almost circular in outline, very thick, lower valve con- 

 cave, the upper valve less concave. Beak of upper valve only 

 slightly projecting beyond that of the lower one. Area 

 broadly triangular, much broader than long, a little more than 

 | as broad as the shell. Ligament groove comparatively shal- 

 low, about as broad as the lateral parts of the area. Anterior 

 margin of muscular impression situated exactly in the middle 

 of the inner surface of the shell (without area). 



Measurements of the type : Length, 169 mm ; breadth, 158 mm ; 

 thickness of lower valve, ca. 35 mm ; breadth of area (lower 

 valve), 5tt mm ; length of area, ca. 25 mm . 



There is no doubt that this species has been seen by others, 

 especially by Darwin, but it has been generally taken for O. 

 patagonica. The chief differences between it and the latter 

 are the following : 1. The outline of the shell is almost circu- 

 lar, in striking contrast to the subtriangular form of O.pata- 

 gonica. 2. In 0. hatcheri the area is comparatively broader 

 and the beaks are not so produced as in O. patagonica, in 

 which the area is only a little shorter than broad. 



O. hatcheri, in its external form, comes very near to 0. 

 maxima Hup. (Philippi, Foss. Terc. Cuart. Chile, 1887, PI. 

 48, fig. 1), but it is at once distinguished by the situation of 

 the muscular impression and the shape of the area, which is 

 decidedly broader in 0. maxima. Moericke (X. Jahrb. Min., 

 etc., Beil. Bd. x, 1896, p. 575), in comparing 0. maxima and 

 patagonica, has already stated these differences. Since he 

 refers to an 0. patagonica from " Santa Cruz," it seems as if 

 he mistook specimens of O. hatcheri for patagonica, and used 

 them for comparison, and not d'Orbigny's figure of the latter. 

 The outline of the true patagonica cannot be compared with 

 that of maxima. 



Localities : The type comes from the Patagonian beds of 

 Santa Cruz (about 250 feet above high tide). Smaller and 

 larger specimens have been found in the Supra-Patagonian 

 beds of Upper Pio Chalia and Shell Gap. One from the latter 

 locality measures: Length, 212 mm ; breadth, 194 mm ; thickness, 

 ca. 50 mm (upper valve). 



