ORTMANN : 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. ARCID^E Dall. 



Gen. ARCA Lamck. 

 30. ARCA PATAGONICA v. Ihering. 



PI. XXV, Fig. 3' 4 . 



1897 -d- 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, i sp.; Arroyo Gio, i 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 : ( i ) 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. 



