W. I. Robinson — Two New Fresh-water Gastropods. 651 



representative of these genera has heen found in beds earlier 

 than late Jurassic, and as the two species described above are 

 closely related to late Mesozoic species, the fossil evidence 

 indicates that the time of the deposition of the new forms was 

 Jurassic and probably middle or late Jurassic. A discussion 

 of the stratigraphy of these beds will be published by Profes- 

 sor H. E. Gregory for the United States Geological Survey. 



EXPLANATION OF FIGURES. 



Fig. 1 a-c. Limnea hopii, sp. nov. 



a and b. Two individuals differing slightly in outline from the 



type specimen (c), twice natural size, 

 c. Apertural view of the holotype, three times natural size. 

 Fig. 1 d aud e. Valvata gregorii, sp. nov. Two views of the holotype, 

 twice natural size. 



Art. XLVII. — The Ordovician Cynthiana Formation ; by 

 Arthur M. Miller. 



As defined by the writer in this paper, the Cynthiana forma- 

 tion of the Cincinnatian series consists of those limestones and 

 shales 40-90 feet in thickness which overlie the Trenton lime- 

 stone as defined by William Linney in his report on Garrard 

 County, Ky.* That author placed as the top member of the 

 Trenton in Mercer and Garrard Counties the " Upper Birdseye 

 Li.nestone," which we now know as the Perryville. Linney 

 considered all of the "Lower Silurian" beds above this 

 horizon in Kentucky as " Hudson," and divided them into 

 three parts, — Lower, Middle, and Upper — each with a thick- 

 ness of about 200 feet. As the upper limit of the Lower 

 Hudson he selected certain layers of " wave-marked limestone" 

 which occur just below sandy beds— the "siliceous mudstone " 

 of Owen. 



M. R. Campbell in his report on the Richmond Quadrangle f 

 accepted the definition of these beds as given by Linney, and 

 applied to them theformation name of " Winchester Limestone." 

 His estimate of their thickness within the limits of the Quad- 

 rangle was 225 feet. 



The writer of this paper, in a report on the Lead and Zinc 

 Bearing Formation of Central Kentucky, % in the manuscript 

 • identified the Cynthiana formation with the Catheys of Ten- 



* Geol. Surv. Ky. , 1 882. + U. S. Geol. Surv. ,1898. 



{Geol. Surv. Ky., 1905. 



