192 



Distinguisliing Characters. — More iie<u-ly circular than 

 R. vanuxeiui ; valves more nearly equal ; pedicle valve flat- 

 tened only near the front: brachial valve but slio-htly more 

 convex. 



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

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

 where it is also rare. 



Khipidomella cyclas. Hall. (Fig. 95.) (Pal. X. Y.. 

 Vol. IV.. p. 52, PI. YII.) 



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

 dinal line, equaling nearly half the width of the shell; a])- 



pressed beaks ; sharply promi- 

 nent radiating stride, which 

 increase by bifurcation and 

 inqjlantation, and have afas- 

 FfG. 95. RhipidomeUa cyclas vetitiai ciculate appearance near the 



and dorsal views, x S (from Hall ). 



front. 



Found in the Hamilton shales, about a foot belo^v the 

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



Measurements. — Largest 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. 



[Ety. : Cjrthos, straiglis ; tluta, the Greek letter 0.] 



(1830: Bull. Soc. Imp. Xaturl. 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 deltidium covering the del thyriam. Teeth not 

 su})ported by dental plates. Brachial valve most convex 

 near the middle, with a narrow hinge area. Cardinal \)vn- 

 cess (|uadrilol)ate as seen from above. Surface covered by 

 slender radiating stria?, which are crenulated by concentric 

 lines. 



