192 



Distinguishing Characters. — More nearly circular than 

 R. vaim.xt'iiii : valves more nearly equal ; pedicle valve flat- 

 tened only near the front; brachial valve but slightly more 

 convex. 



Found rarely in the lower five feet of the Moscow shale, at 

 Sections 4 and 5 ; in the Encrinal limestone, at Section 5, 

 where it is also rare. 



Rhipidomella cyclas. Hall. (Fig. 95.) (Pal. \. Y.. 

 Vol. IV.. p. 52, 1M. VII.) 



Distinguishing Characters.— Small size of adult; long car- 

 dinal line, equaling nearly half the width of the shell; ap- 



pressed beaks; sharply promi- 

 nent radiating stria 1 , which 

 increase by bifurcation and 

 implantation, and have afas- 

 fig. 05. Rhipidomella eyOas Central ciculate appearance near the 



and dorsal views, x ',' (from Hall ). 



front. 



Found in the Hamilton shales, about a foot below the 

 Trilobite layers, in Avery's Ravine (one specimen). 



Measurements. — La rgest specimen (Hall): length, five- 

 tenths of an inch + ; width, six-tenths of an inch + ; length 

 of area, thirty-three hundredths of an inch ( + ). 



Genus ORTHOTHETES. Fischer de Waldheim. 



[Etv. : Ortltos, straighs ; theta, the Greek letter 0.] 



(1830: Bull. SOC. Imp. Nat ml. d. Moscow. T. I., p. 375. 1892: Pal. X. Y.. 



Vol. VIII., Pt. I., p. 253.) 



Shell varying From plano-convex to bi-convex, sometimes 

 becoming concavo-convex with age. Pedicle valve most 

 convex about the beak, which often tends towards irregular 

 growth; cardinal area well developed, with a thick, more 

 or less convex deltidinni covering: the del thy rium. Teeth not 

 supported by dental plates. Brachial valve most convex 

 near the middle, with a narrow hinge area. Cardinal pro- 

 cess quadrilobate as seen from above. Surface covered by 

 slender radiating striae, which are crenulated bv concentric 

 lines. 





