FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 163 



The surface is marked by strong concentric striae, which are interrupted and 

 irregular from the numerous nodes projecting from the shell, and extended 

 into long tubular spines. 



This shell, in full grown specimens, attains sometimes a length of two inches 

 and a half, and its entire surface is covered with strong, tubular spines, which 

 are sometimes two inches long, as may be seen in the inclosing rock. In the 

 specimen figured on plate 23, figure 2, all the spines are preserved to a certain 

 extent ; some measure more than half an inch. These shells were found at the 

 falls, imbedded in soft limestone ; the shells themselves were entirely silicified. 

 By the use of muriatic acid, the limestone envelope was gradually removed, 

 and any portion of the shell or spines, whenever it was sufficiently freed from 

 the matrix, was then carefully coated with paraffine, to prevent the acid from 

 further acting upon these cleared portions. By this process, in which very 

 diluted acid must be used, and which requires often several days close watch- 

 ing, in order to protect the exposed parts in time from the further action of 

 the acid, the most excellent specimens in shells, crinoids, and especially in 

 coral, are obtained. In the collection of my co-laborer and friend, Major Wm. 

 J. Davis, the author of "Kentucky Fossil Corals," the most exquisite and 

 valuable specimens of which that cabinet is so rich, and, therefore, unparalleled 

 by any other collection in this country, are produced by the careful application 

 of muriatic acid and paraffine. The specimens of this species, found in the 

 clay, are mostly exfoliated, and show the places of the spines by moderately 

 elevated nodules, as may be seen in the figures 5, 6 and 12 ; the last one is that 

 of a young specimen. 



Formation and Locality. Occurs in the upper strata of the Corniferous limestone, just above the 

 hydraulic cement rock, at the Falls of the Ohio, on the Kentucky side of the river; the layers containing 

 them are only exposed at a very low stand of the river. In the clay, exfoliated shells are found in differ- 

 ent places in Jefferson county, Ky., and in Clark county, Ind. 



Platyceras dumosum, var. rarispinum. HALL. 



Plate XXIII., figures 7 and 8. 



Platyc. dumosum, var. rarispinum. Hall. Desc. of New Sp. of Foss. 1861. 

 Platyc. dumosum, var. rarispinum, Hall. 15th Reg. Rep., p. 38 1862. 

 Platyc. dumosum, var. rarispinum, Hall. Illust. of Dev. Foss., pi. 5 1876. 

 Plat, dumosum, var. rarispinum, Hall. Pal. N. Y., Vol. 5, pt. 2, p. 16 1885. 



Shell only of medium size or below it ; sub-ovate ; apex closely incurved 

 and enrolled for about one and one-half volutions, and in some shells the volu- 

 tions are contiguous nearly to the aperture, as seen in figure 8, plate 23. For 

 one volution and a half or three-fourths the shell is slender and only gently 

 enlarging ; after that the body- whorl expands more rapidly and becomes mod- 

 erately ventricose, depressed on the dorsum, and the left side sometimes 



