PALEONTOLOGY ZORRITOS FORMATION 157 



Shell large for the genus, thin, fragile, subequilateral. 

 Margins not preserved in specimens studied, but casts show 

 them to be probably evenly rounded, the ventral margin 

 moderately arcuate. Umbones pointed, not prominent, 

 terminal. A barely perceptible posterior keel, followed 

 behind by a shallow sulcus, breaks the otherwise evenly 

 rounded surface, which is only slightly convex; in some 

 specimens the central surface is almost flat. Sculpture of 

 fine concentric growth-lines. Escutcheon long and narrow; 

 lunule poorly marked or absent. Hinge strong for the genus, 

 the anterior cardinal of the left valve being prominent and 

 clearly bifid, and the socket behind it large and well marked ; 

 otherwise the hinge is normal. Inner shell unknown. 

 Length, 67; height, 42; (semi) diameter, 9.5 mm. 



This species is very similar to a number from the Miocene 

 of Panama, Dominican Republic, and Europe. It is close to 

 T. aequiterminata Brown and Pilsbry, 178 from Gatun, but 

 its ventral margin is rounded instead of straight, as in 

 aequiterminata. Further points of difference or similarity 

 can not be determined on account of the incompleteness of 

 the Gatun form. It is similar in shape to T. maoica 

 Maury, 179 from the lower Miocene on the Dominican Re- 

 public, but is less inflated, the , sculpture is less prominent, 

 and the posterior fold is not so well marked. T. dbaoica 

 Maury 18 is also similar, but is likewise more inflated, and 

 the hinge is weaker. 



The hinge, which is distinctive, is closely similar to none 

 described from the Americas, but is practically the same 

 as that of two European species which appear in the Tor- 

 tonian T. strigosa, Gmelin 181 and T. planata Linnaeus. 132 



178 Proc. Ac. Nat. Sc. Phila., vol. 64, p. 517, pi. 26, fig. 5, 1912. 



179 Bull. Amer. Pal., vol. 5, No. 29, p. 223, pi. 38, fig. 5, 1917. 



180 Idem., p. 223, pi. 38, fig. 10. 



m Hoernes, M., Abh. k.k. geol. Reichs., Bd. 4, p. 83, pi. 8, 

 figs. 8a, b, c, 1870. 

 182 Idem., p. 84, pi. 8, figs. ;a, b, c. 



