82 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 



Patagonica, although the outline is more elongate, and the number of 

 hinge teeth appears to be smaller. In outline, as well as the lack of cren- 

 ulations, the Pliocene and Recent N. tenuis (Mont.) (see Wood, 1856, p. 

 84, pi. 10, f. 5) still more resembles our species, but N. tenuis is much 

 thinner, and has distinctly a convex anterior dorsal margin. Thus we 

 may say that, if there are any relations of the Patagonian shell to known 

 forms, they are with Oligocene and Pliocene species. 



In so-called "Eocene" (or Miocene?) beds of Victoria, South Australia, 

 and Tasmania, we have N. tumida Ten.-Wood (Tate, 1886, p. 127, pi. 6, 

 f. 6) as a representative of N. patagonica. 



23. NUCULA RETICULARIS Ortmann. 



PI. XXV, Fig. 8"' 6 . 

 1900 R. ret. Ortmann, in: Amer. Journ. Sci., v. 10, p. 371. 



Shell small, moderately convex and moderately thick, subovate, oblique ; 

 posterior and anterior dorsal margins slightly convex, ventral margin 

 strongly arcuate. Surface with very fine concentric ribs, which are, espe- 

 cially near the posterior extremity, irregular and bifurcate. These ribs 

 are crossed by still finer radial striae, which give a beautifully reticulated 

 appearance to the shell. Ventral margin finely crenulated on inner side. 

 Hinge teeth fine, both parts of the series forming an obtuse angle ; ante- 

 rior part with ca. 9, posterior with ca. 18 teeth. 



Measurements: Length, 7.5 mm; height, 6 mm. 



Remarks: The radial striae are sometimes obliterated by fossilization 

 and indistinct, but are recognizable in almost all individuals on closer 

 inspection. 



Record of specimens : Mouth of Santa Cruz River, i valve; Mt. of Obser- 

 vation, upper horizon, 16 valves. 



Affinities : The small size and sculpture characterizes this species suffi- 

 ciently. This peculiar sculpture, however, brings our species very near 

 to the Oligocene N. chasteli Nyst (see Sandberger, 1863, p. 342, pi. 28, f. 

 7), from Germany. In N. chasteli the same dichotomy of the concentric 

 ribs is present, but much more strongly pronounced, and the external form 

 is different : it is triangular or hatchet-shaped. 



