PALEONTOLOGY ZORRITOS FORMATION 173 



margin short and biangulate. Umbones anterior, not prom- 

 inent. Sculpture of fine concentric lines. Surface ovately 

 convex, the rostral portion flat, with the rostral keel obso- 

 lescent, and the intercarinal surface concave. Cardinal angle 

 not acute. Posterior and pointed, although somewhat bluntly 

 so. Lunule and escutcheon absent. Pallia! line impressed. 

 Teeth unknown. Length, 9.5; height, 6; (semi) diameter, 

 2.5 mm. (right valve) ; diameter of entire shell, 4.5 mm. 



This species is similar in general outline to C. propinqua, 

 of the Variegated fauna, but differs as noted in the discus- 

 sion of that form.. It is most closely related to C. caimitica 

 Maury, 198 of the Santo Domingan Lower Miocene, which 

 differs in being slightly shorter, with more nearly central 

 beaks ; it is not as much produced posteriorly ; the first pos- 

 terior carina is sharper, and turns upward in the left valve, 

 whereas that of fabiformis is nearly straight. Maury gives 

 no description worth noting, and her figures are insufficient 

 owing to high lights ; but such features as are notable show 

 the two forms to be probably of different species, though 

 very close. It is similar in general outline to C. sphenia 

 Dall, 199 from the Chipola of Florida, but the sculpture of 

 sphenia is much coarser. C. kaghriana Dall, 200 from the 

 Orthaulax zone of Ballast Point, Florida, is also similar, 

 but differs in being shorter and in having coarser sculpture, 

 the interspaces between the ribs being wider. Corbula 

 hatcheri Ortmann, 201 from the Lower Miocene of Patagonia, 

 is very similar, but is shorter, and its ventral margin ascends 

 more abruptly to the posterior marginal angle. The ventral 

 surface of hatcheri is very similar to that of fabiformis. 



Lower Zorritos. Quebrada north of Boca Pan. 



198 Bull. Amer. Pal., vol. 5, No. 29, p. 233, pi. 39, figs. 18, 19, 1917. 

 109 Trans. Wag. Inst, vol. 3, pt. 4, p. 847, pi. 36, fig. 10, 1898. 

 200 U. S. Nat. Mus. Bull. 90, p. 155, pi. 21, figs. 4, 5, 1915. 

 ^Princet. Exp. Patag., vol. 4, p. 151, pi. 30, figs. 4a, b, c, 1901. 



