452 GEOLOGY OF OHIO. 



Specimens of this species, presenting all the specific features of the 

 typical forms from the Hamilton beds of New York, are found at Fish- 

 inger's Mills, and at Scioto Station, in beds of limestone above the 

 "Bone-bed." 



The general form is transversely elliptical, a little more than half as 

 long again as high, the valves usually compressed somewhat in the direc- 

 tion of bedding, but still moderately convex and extremely Unio-like in 

 their general expression. The body of the valves is marked by the 

 characteristic oblique rib and furrow passing from the beak to the postero- 

 basal margin, somewhat modifying its border; also by numerous concen- 

 tric folds or Crinkles parallel to the margin of the shell, and marking 

 stages of growth. These wrinkles are usually well marked on the ante- 

 rior end of the shells, and become faintly marked or obsolete posterior to 

 the oblique furrow, and on the rather wide posterior cardinal slope. The 

 hinge-line is nearly straight and shorter than the length of the shell be- 

 hind the beaks, causing an oblique truncation of the posterior end above 

 the longest point of the valve. Beaks large, tumid, situated well forward 

 on the valves and enrolled. The oblique ridge is generally more or less 

 nodose from the crossing of the concentric folds of the shell. 



This species has always been considered a very characteristic and 

 well-marked Hamilton type; and its occurrence in layers above the horizon 

 of the "Bone-bed," and not below, is very significant. 



FOSSILS OF THE ERIB SHALES. 



There appears to be no question regarding the equivalence of the 

 Erie shales of Ohio with the Portage and Chemung groups of New York, 

 and the palasontological features of these latter formations are so well 

 known, and so marked, that there ought to be no doubt as to their geo- 

 logical position. Their strati graphical relations also to the Catskill group, 

 the American equivalents of the Old Red Sandstone of England, which 

 is considered as typical Devonian, would apparently leave no doubt as to 

 their place in the geological record, or to the zoological age to which they 

 should be referred. From these considerations ' I have considered 

 the following fossils from the Erie shales, as of Devonian age, an opinion 

 for which I alone may be held responsible. The group, taken as a whole, 

 are of special interest on account of the Crustaceans; while the other 

 forms associated with them are sufficiently characteristic to show their 

 stratigraphical relations. 



MOLLUSCOIDA. 

 Genus DISCINA Lamarck. 

 Discina humilis. 

 Plate VIII, figs. 1 and 2. 

 Discina humilis Hall, Pal. N. Y., vol. 4, p. 16, pi. 2, fig. 18. 



A crushed and fragmentary specimen of this species, but quite too 

 imperfect for illustration, has been detected in one of the nodules from 



