ORTMANN : TERTIARY INVERTEBRATES. 139 



65. VENUS VOLCKMANNI Philippi. 



PI. XXVIII, Fig. 3-. 



1887 V. v. Philippi, Tert. & Quart. Verst. Chiles, p. 121, pi. 14, f. 9. 

 1897 ^ v- var - argcntina v. Ihering, in : Rev. Mus. Paul., v. 2, p. 252, 



pi. 7, f. 45- 

 1899 V. v. v. Ihering, in: N. Jahrb. Miner., etc., v. 2, p. 20. 



Shell suborbicular, very convex; posterior dorsal margin slightly con- 

 vex, forming an obtuse angle with the posterior margin, which forms, with 

 the ventral margin, part of an almost regular circle. Apex between l / z 

 and % of the length. Area indistinct, nymphae immersed. Lunula cor- 

 date, elevated in the middle. Surface with rather widely distant, elevated 

 lines, and distinct radiating striae. 



Measurements: Length, ca. 73 mm ; height, 70 mm ; diameter, 27 (x 2). 

 Another individual: length, 55 mm; height, 51 mm; diameter (double), 

 35 mm. In the latter, the apex is 15 mm from the anterior end. 



Remarks: Philippi gives the position of the apex as 2 ^ = ca. jof the 

 length, v. Ihering as ^ = ca. % . This difference is apparently in part due 

 to the different position given to the shell, as the regular outline of the 

 shell renders it difficult to give a uniform position to different specimens. 



The form from Santa Cruz is hardly distinguishable as a variety from 

 that from Chili (in 1899 v. Ihering mentions typical individuals from Santa 

 Cruz). The convexity of the dorsal margin is variable, and the anterior 

 end (below the lunula) is more or less produced in different individuals : 

 There are specimens from Santa Cruz completely agreeing with Philippi's 

 figure in this respect. 



The cast from Shell Gap does not show any surface markings, but agrees 

 in form. The casts from Lake Pueyrredon, however, show distinct remains 

 of the cancellations. The cast from the mouth of Santa Cruz River shows 

 the impression of the cardinal teeth : they are those of a true JSenus. 



Record of specimens : Mouth of Santa Cruz River, i double cast (near 

 high water mark), i double valve, 2 isolated valves (250' above high tide). 

 Shell Gap, Rio Chico, upper horizon ; i cast. Lake Pueyrredon, base of 

 Tertiary ; 3 casts. Lake Pueyrredon, 600' above base ; 2 casts. 



Distribution: Patagonian beds of Santa Cruz (v. Ih.) ; Navidad beds 

 of Chili: Navidad, Tubul, Millanejo and Lebu (Phil.). 



