ORTMANN : TERTIARY INVERTEBRATES. 7 1 



or less elevated fold, corresponding to a mesial sinus in the larger valve. 

 Surface ornamented by sharp plaits, about 25 in number, 4 to 7 of which 

 are in the fold and sinus ; these plaits become indistinct and disappear in 

 the upper part of the shell. 



Measurements: Length 15, 16, 18, 20, 20, 22, 24, 21, 21 mm. 

 Width 14, 20, 1 8, 21, 23, 20, 24, 24, 28 mm. 



Remarks: This species comes very near to R. nigricans (Sow.) (see: 

 Davidson, 1852, p. 81, pi. 14, f. 30, 31 ; Suess, 1864, p. 60, pi. 14, f. 5; 

 Hutton, 1873, p. 37; Davidson, 1887, p. 169, pi. 24, f. 16-19). The 

 only difference of our fossil Patagonian form from the New Zealandian 

 is the tendency of the plaits to disappear toward the umbo of both 

 valves. 



V. Ihering compares this species with R. nigricans, and says that it 

 differs in the following particulars: (i) In the more triangular and less 

 transverse shape ; (2) in the straighter and more pointed beak; (3) in 

 the smaller foramen. That the first character has no value is shown by 

 our material, in which the outer form varies considerably, as is seen by a 

 glance at our figures and the measurements given above ; and the same 

 is true of the second character. As to the foramen, the figure of v. Iher- 

 ing does not show a smaller foramen than those of Davidson and Suess, 

 and (although our specimens show, as a rule, a foramen a little smaller 

 than in the figures quoted) this difference is very slight. In a letter, v. 

 Ihering again calls my attention to the difference in the foramen, and 

 compares that of R. plicigera to that of R. psittacea. In the latter spe- 

 cies (judging from specimens collected by myself in Inglefield Gulf, North 

 Greenland) the foramen is narrow, elongated, and the deltidial pieces on 

 each side of it are elongated-triangular, while in R. nigricans (according 

 to Davidson) the foramen is more rounded and the deltidial pieces are 

 broadly and almost equilaterally triangular. Examining our fossil form 

 in this respect, it agrees with R. nigricans, and not with R, psittacea. 

 But there seem to be variations even in R. nigricans, as is seen in David- 

 son's figures, and confirmed by Suess's figures, which have a distinctly 

 elongated foramen. 



Record of specimens : Las Salinas, i sp. ; Shell Gap, upper hori- 

 zon, i sp. ; east end of Lake Pueyrredon, i sp. ; high bluffs, S. W. 

 of Lake Pueyrredon, 2 sp. ; Lake Pueyrredon (Rio Tarde section), base, 

 35 sp. 



