100 PALAEONTOLOGY OF KENTUCKY. 



ervation, shows a perforation in the beak, opening below into an angular 

 space, which has been occupied by the beak of the other valve, and thence 

 communicating with the main cavity of the valve. The base of the fissure is 

 margined on each side by a strong tooth, which extends in strong dental plates 

 to bottom of cavity, and these are often continued in a thickened ridge bor- 

 dering the muscular impression. The imprints of the adductor muscles are 

 opposite the base of the dental plates, and below and on either side are the 

 imprints of the broad divaricator muscle. In dorsal valve, cardinal process is 

 broad and strong ; crural bases somewhat widely diverging and center abruptly 

 depressed. The teeth sockets are large, and supported by strong lamellar cal- 

 losities, which extend along inner side of the valve nearly parallel to the ex- 

 terior margin. The muscular imprints are divided by a low distinct septum. 

 This shell presents considerable variation in its form and general aspect, 

 though always preserving its unmistakable character. In young and half 

 grown shells, where the hinge-line is but little extended, it has a general tri- 

 angular form. When the hinge-line becomes extended, and the sides nearly 

 straight, with a moderately curving base, it is quadrangular. Some specimens 

 assume the shape of a hexagon, others of a pentagon. In some individuals 

 there is a slight elevation in center of sinus, but this is of rare occurrence, 

 oftener the sinus contains a central rounded depression, margined by a slight 

 elevation, which extends from beak to front. The beak of the ventral valve 

 is sometimes not closely pressed upon the dorsal umbo, but in most specimens 

 is such the case, and oftentimes to such a degree that the apex is incurved into 

 the groove of the mesial fold. On the dorsal valve we find two considerable 

 elevations, one on each side of the mesial fold ; they start from the cardino- 

 lateral margins, near to which they have their maximum elevation, gradually 

 decreasing until they disappear in the surface of the valve a little above its 

 middle ; their direction is not quite parallel to the mesial groove, but is deflect- 

 ing outwards. (Copied from Hall's description, with many alterations of my 

 own.) 



Formation and Locality. Found in the upper strata of the Corniferous group surrounding the 

 Falls of the Ohio, in Kentucky and Indiana, where fractions of this species are pretty abundant in some 

 localities, but fine and well preserved specimens of the whole shell, as well as of single valves, which are 

 found, are exceedingly rare. My cabinet contains some exquisite examples of this species. The fossils ot 

 the Corniferous strata, from the neighborhood of the Falls, are, on the Indiana side of the river, generally 

 more numerous, and in the average better preserved, than those found in Kentucky. The little town 

 Charlestown, in Clarke county, Indiana, two or three miles off the river, is about the center of one of the 

 richest fields of the Devonian formation, which has furnished a great many cabinets with very choice speci- 

 mens. A day's rambling in the washes of the fields around Charlestown, after several days' hard rain, is 

 a real treat to any geologist, and never fails to fill his basket with fine shells, beautiful corals, and some- 

 times, but not very often, with rare crinoids. 



