Pal. anacrata. Length & width as 5 to 7, hinge line nearly equals the width of shell. Dorsal valve flat with a slight depression down the centre. Ventral valve depressed convex, an undefined elevation extending from umbo towards its front, & sometimes quite to the margins of the shell. Valves thin. Rhyachtherma dentatum. Published as a Trenton species from Trenton NY, But the types are referred by Whitfield & Hurley to Cin. O. The typical specimens are quite commun at the quarries in Hamilton Butler or Bluff. It has not been found associa- ted with O. anacrata, but layers, and, as far as known, lies a large stratocing about 50 feet above the latter, and extending to nearly the top of the exp cause of the lower shell series. The largest speci- men that I have seen was found near Byford in Butler G. Var. (= Fig. 6A 2) anacrata. It may be readily distinguished from O. which it has been cited as by the following characters: 1. It is our aller, strae cast or finer (2) anacrita being more and valve and on ventral side a the ventral valve more distinct and better defined, greatest convexity of the ventral when none central.