ORTMANN : TERTIARY INVERTEBRATES. I 1 7 



40. PECTEN GEMINATUS Sowerby. 



PI. XXIII, Fig. 2". 



1846 P. gem. Sowerby, in: Darwin, Geol. Observ. S. Amer., p. 252, pi. 



2, f. 24. 

 1897 P- quemadensis v. Ihering, in: Rev. Mus. Paul., v. 2, p. 228, pi. 6, 



f. 38. 



1899 P- fissicostalis v. Ihering, in: N. Jahrb. Min. Geol. Pal., v. 2, p. ,n 

 pi. i, f. i. 



1900 P. geminatus Ortmann, in: Amer. Journ. Sci., v. 10, p. 379. 



Both valves almost equally convex, the right one a little less so. Outline 

 suborbicular in old individuals, subtriangular in younger ones. Anterior 

 ears considerably larger than the posterior, that of the right valve with a 

 deep byssal sinus. Outer surface of right valve with from 2030 strong 

 ribs, arranged somewhat irregularly in pairs. All the principal ribs dis- 

 tinct down to the lower margin, but on the lateral parts of the shell they 

 become less distinguishable from the smaller (secondary) ribs. Intervals 

 between the principal ribs a little broader than the ribs, occupied by from 

 i to 5 secondary ribs or striae. All ribs covered with squamae. In very 

 old individuals the number of the intermediate ribs increases to 7, and 

 those adjoining the principal ribs grow larger, giving a fasciculate appear- 

 ance to the principal ribs. The median secondary rib in each interval is 

 usually a little stronger than the rest. In the left valve the character of 

 the ornamentation is practically the same, but the geminate character of 

 the principal ribs is altogether lacking. 



Measurements: Height, 105, 85, 35 mm. 

 Width, 107, 80, 28 mm. 



Remarks: The character of the geminate ribs is present only in the 

 right valve, and distinct only on the median part of it ; near the lateral 

 margins principal and secondary ribs are hardly distinguishable. Young 

 individuals, of about the size of Sowerby's figure, ca. 30 mm, show only 

 the principal ribs and a few single striae in the intervals, so that the total 

 number of ribs is only about 20 to 24. In larger individuals, of the size 

 of v. Ihering's figure of P. quemadensis, ca. 35-40 mm, the intermediate 

 ribs become more numerous, and especially the ribs near the lateral mar- 

 gins increase in number, so that we may count 24-30 ribs. In still larger 



