STANTON : THE MARINE CRETACEOUS INVERTEBRATES. I I 



PELECYPODA. 



OSTREA TARDENSIS Sp. HOV. 

 PI. I, Figs. I and 2, and PI. II, Figs, i and 2. 



Shell large, massive, subtriangular, or more or less crescentic in outline ; 

 lower valve very thick and very convex, obscurely carinate, with the beak 

 more or less twisted laterally, but not distinctly coiled, the lower third of 

 the valve also laterally curved, so as to give the shell its crescentic shape ; 

 upper valve thinner, flat or somewhat concave, with the beak more nearly 

 straight and the other end curved to fit the lower valve ; surface of both 

 valves with rather coarse concentric lamellae or imbrications, though on 

 most of the specimens in the collection these are obscured by weathering. 

 Some individuals also show obscure radiating plications. The pit or 

 groove for the ligament is very large and broad and only slightly curved 

 in both valves. 



An average specimen measures 150 mm. from beak to base and 90 

 mm. in greatest breadth at right angles to that line. Convexity of the 

 two valves 53 mm. 



The eight specimens in the collection show some variations in form 

 and proportion, but not more than species of Ostrea usually show. The 

 curved form gives it the appearance of an Exogyra, but the beaks, espe- 

 cially of the upper valve, lack the spiral form characteristic of Exogyra. 



The fact that Exogyra couloni Defrance has been reported from several 

 localities in South America led to the comparison of that species with our 

 Patagonian form. While it is true that some extreme varieties that have 

 been figured as belonging to that very variable European Lower Creta- 

 ceous species somewhat resemble the Patagonian shell in form and gen- 

 eral appearance, it does not seem to me that they can be identical. O. 

 tardensis is certainly not at all like the typical and ordinary forms of E. 

 couloni. Behrendsen 1 reports the occurrence of E. couloni at "Arroyo 

 Pequenco between Rio Salado and Rio Malargue, not far from the Villa 

 Beltran," Argentine Republic, and states that it is present in almost all 

 the forms represented by d'Orbigny and Pictet, but agreeing especially 

 with the figures given by Bayle and Coquand 2 of specimens from Chili. 



1 Zeitschr. Deutsche Geol. Gesellsch., Bd. 43, p. 419, 1890. 



2 Mem. Soc. Geol. de France, 2d Ser., t. IV, p. 37, pi. 7, figs. I and 2, 1851. 



