ORTMANN I TERTIARY INVERTEBRATES. 
93 
more striking is the fact that this identical type is represented in the 
Pareora beds of New Zealand and in Australia. 
Fam. ARCIDxE Dali. 
Gen. ARCA Lamck. 
30. Arca patagonica v. Ihering. 
PI. XXV, Fig. 3 a A 
1897 ^ 4 - P a t- v - Ihering, in: Rev. Mus. Paul., Vol. 2, p. 235, pi. 4, f. 23, 
pi. 5, f. 30. 
Shell elongate, anterior part short, rounded, posterior elongated, 
obliquely truncated. Ventral margin almost straight and parallel to the 
upper margin (hinge-line). A blunt, but distinct carina runs down from 
the apex to the posterior ventral angle. Apices remote from each other, 
area broad, concave, with a large sulciferous rhombus. Surface of shell 
with radiating ribs, anterior ribs stronger, median ribs finer, and on the 
posterior part of the shell, above the carina, again 3-5 (according to age) 
stronger ribs. All ribs noduloso-imbricated by concentric lines crossing 
them. In older parts of the shell (near the apex) the ribs are much 
crowded, but they become more distant on approaching the ventral 
margin, and finer ribs develop in the intervals. In old individuals, near 
the ventral margin, from 2 to 4 finer ribs are found between the 
stronger ones. 
Our largest individual measures : Length, 28 mm ; height, 20 mm ; 
diameter, 10 (x 2) mm. 
Records of specimens : Mouth of Santa Cruz River, ca. 30 sp. ; Mt. of 
Observation, upper horizon, 1 sp.; Arroyo Gio, 1 cast. 
Distribution: Jegua quemada, Suprapatagonian beds (v. Ih.). 
Affinities: A. oxytropis Philippi (1887, p. 188, pi. 37, f. 6) from the 
Navidad beds of Lebu, Chili — although very incompletely known — seems 
to be closely allied to this species. In A . oxytropis the following char- 
acters agreeing with A. patagonica are known: (1) the elongated and 
narrow form, (2) the oblique truncation of the posterior end, (3) the ridge 
running across the valve, (4) the existence of a few (3) ribs above this 
ridge. 
