18 



Q 



(Cashaqua) shales, at Section 1 ; in the Genesee shale, at 

 Sections 1 and 2, and on the Lake Shore; in the Styliolina 

 band, at Sections 1, 2, 3; in the Demissa bed, ;ii Sect ion 5; 

 and between twelve and fourteen feet below the Fncrinal 

 limestone, at Section 7. 



Measurements of a very large individual were 6 mm. 

 long- and 3.5 mm. wide. Usually they are much smaller. 



Genus ORBICULOIDEA. D'Orbigny. 



[Ety. : Orbicula, a genus; aides, like.] 



(1850: Prodr. de Paleont, T. I., p. 44. 1893: Pal. X. V., 

 Vol. VIII., Pt. L, p. 120.) 



Shell varying from nearly circular to almost elliptical in 

 outline, the valves unequal; apices eccentric; lower (pedicle) 

 valve hat, or nearly so; a narrow pedicle furrow passes 

 backward from just behind the beak, and ends near the 

 margin in a short tubular sipho, which penetrates the 

 shell and emerges on the inner sides, near the margin ; upper 

 (brachial) valve conical, apex directed backwards; surface 

 marked by concentric lines of growth; shell substance 

 partly corneous. 



Orbiculoidea media. Hall. (Fig. 82.) (Pal. X. Y.. Vol. 

 IV., p. 20, PI. II. Discinn media,. ) 



Distinguishing Characters. — Size; broadly elliptical or 

 nearly circular outline ; depressed brachial valve, with apex 

 at one-third the distance from the pos- 

 terior margin; pedicle furrow commonly ,. 

 in the longitudinal axis; fine regular 



Surface Stride. FiG , g2 Orbiculoidea 



Found in the upper Moscow shales ^; n Btfg 

 (transition shales), at Section 3; in the HallK 

 middle Moscow (0. media) bed, at Sections 4 and 5; at ten 

 feet below, and between fourteen and seventeen feet below 

 the Encrinal limestone, at Section 7; and in the Modiomor- 

 pha subalata bed, on the Lake Shore. 



Orbiculoidea doria. Hall. (Fig. 83.) ( Pal. X. Y.. Vol. 

 IV., p. 19, PI. II.) 



