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, 18, 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: (1) 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- 
ino' does not show a smaller foramen than those of Davidson and Suess, 
O 
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, 1 sp. ; Shell Gap, upper hori- 
zon, 1 sp. ; east end of Lake Pueyrredon, 1 sp. ; high bluffs, S. W. 
of Lake Pueyrredon, 2 sp. ; Lake Pueyrredon (Rio Tarde section), base, 
35 sp. 
