ORTMANN : TERTIARY INVERTEBRATES. 115 



aspect from large individuals, where the lower half of the valve is covered 

 completely with strong radial ribs. V. Ihering's figure 48 (right valve, 

 Proximiis] represents an individual in which the intermediate ribs have 

 not begun to develop. This species cannot be the P. centralis of 

 Sowerby, as will be seen below. 



Record of specimens : Mouth of Santa Cruz River; 5 double, 9 right, 4 

 left valves. 



Distribution: Jegua quemada, Suprapatagonian beds (v. Ih.). 



Affinities : A most closely allied form is P. caracolensis Steinmann 

 (1881, p. 254, pi. 14, f. 10) =P. simpsoni Philippi (1887, p. 210, pi. 46, 

 f. i ) from the Navidad beds of Chili, but in P. caracolensis (according to 

 Philippi's figure) the principal folds are more numerous and narrower, 

 and the ribs are less numerous. 



P. athleta Zittel (1864, p. 49, pi. 10, f. i) from the Oligocene (Oamaru, 

 Hutton, 1873, p. 32) of New Zealand is also closely allied. It differs in 

 the same characters and seems hardly distinct from P. caracolensis. 



As to the relation to P. caloosaensis Dall see below (under next 

 species). 



38. PECTEN PR^ENUNCIUS v. Ihering. 



PI. XIX, Fig. 2 a '. 

 1897 P- Pr<%n. v. Ihering, in: Rev. Mus. Paul., v. 2, p. 230. 



Similar to P. proximus in the unequal valves, and the large folds 

 (5-7). The differences are : Right valve with a slight byssal sinus. No 

 radial ribs, but only fine radial striae. 



Our specimens are smaller than P. proxiimts : Height, 63 mm; width, 

 54 mm. 



Remarks: The characters given above seem to agree with v. Ihering's 

 species, and moreover, through the kindness of Dr. v. Ihering, I possess 

 a lead-pencil sketch of P. prcsnuncius, which removes all doubt as to the 

 identity of our specimens with this species. 



In the two left valves from San Julian the radial folds are quite strong, 

 especially in the one figured (pi. XIX, fig. 2*). The right valve is small, 

 and the folds are of medium size. Of the specimens of Santa Cruz, one 

 of the right valves agrees completely with that from San Julian ; in the 

 other one the radial folds are very slight (pi. XIX, fig. 2"). The left 

 valve from Santa Cruz shows hardly any traces of folds, and is almost 

 perfectly flat. 



