210 PALAEONTOLOGY OF KENTUCKY. 



Valves regularly convex, somewhat regularly gibbous in the middle. Beaks 

 a little anterior to the middle, small, appressed, and closely incurved, rising 

 but little above the hinge-line. Umbonal slope denned above by a depression 

 extending from the beaks to about the middle of the posterior extremity, dis- 

 tinctly limiting the post-cardinal slope of the valves. 



Test thin; surface marked by fine concentric striae, which are sometimes 

 aggregated into fascicles at irregular distances. 



Ligamental groove narrow and elongate. Posterior muscular impression 

 just within the post-cardinal margin and below the ligamental groove. Pallia! 

 line parallel with the basal margin, marked in the cast by a row of elongated 

 nodes, which are the terminations of low ridges from above. Interpallial area 

 pustulose on the cast. (Hall.) 



This species "resembles Lucina proavia of Goldfuss to such an extent that it 

 is impossible to distinguish the German species from the American. I have in 

 my possession a large number of perfectly preserved specimens of our shells, 

 and two equally well preserved specimens of Lucina proavia from the old 

 country, but so far none of my geological friends has been able to pick out 

 the foreign species. 



Prof. Hall says the hinge of the German shell is much more declining, and 

 the anterior end is more elevated and sub-auriculate, as shown in Goldfuss' 

 figures. If such a difference is shown in the figures, which I do not doubt, 

 it may be that they were taken from a peculiarly formed specimen which did 

 not represent the average form of the species, or the figure may not be a cor- 

 rect copy of the specimen from which it was taken. Small differences, as 

 those marked out by Prof. Hall, taken from figures, should always be con- 

 sidered doubtful, and never used without other evidences to establish a new 

 species. I have not the least doubt that, in the course of time, the identity 

 of P. elliptica and L. proavia, will be acknowledged by all American geolo- 

 gists, and the fossil will then be known under the name of Paracyclas proavia. 

 The generic name Lucina, has to be dropped for other reasons. 



Formation and Locality. In the Corniferous rocks of Kentucky and Indiana, around and at the 

 Falls of the Ohio, where specimens of exquisite beauty and perfection, with the exterior shell as well as 

 internal casts are found. 



Paracyclas elongata. u. SP. 



Plate II., figure 8. 



This shell resembles very closely P. lirata of Conrad, but it differs from it 

 greatly in form, so much so, that any one must distinguish the two species at 

 the first glance. While P. lirata has almost the shape of a regular circle, 

 this shell has the form of an ellipse, in which the larger axis exceeds the 



