ORTMANN : TERTIARY INVERTEBRATES. 
I 17 
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. ,11 
pi. 1, f. 1. 
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 20-30 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 
1 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 
