May, 1841.] 19 



foot of the Falls, and is found in many parts of the western coun- 

 try, is probably the equivalent of the Marcellus Shale of New 

 York. This is an excellent landmark, as there is no other for- 

 mation in the west that can be easily mistaken for it. The situation 

 of this slate at the Falls has been misunderstood in some instances, 

 and described as underlying, or beneath the limestone. 



"The dividing line between the upper and lower Silurian 

 groups, (Wenlock Shale and Caradoc rocks,) appears to have no 

 distinct lithological demarcation in our western formations; yet 

 this line will probably be found to occur in the upper series of 

 the Cincinnati and Madison Blue Limestones and Marls." 



Mr. T. A. Conrad submitted a description of three new 

 species of Unio, from the rivers of the United States, viz.: 



1. Unio perplicatus. Obtusely subovate, very ventricose; rather 

 thick, with about 12 oblique, profound plica?, those behind the 

 umbo recurved ; ligament margin greatly elevated ; posterior 

 superior margin slightly concave, oblique, extremity truncated ; 

 epidermis blackish-brown, apex erobed ; within tinged with 

 purple; cardinal teeth direct, prominent, sulcated. (Length 

 2.4; height 1.3; diameter 1.9.) 



This species is most nearly related to U. costatus, (Raf.) but 

 differs in being far more ventricose, and has very prominent urn- 

 bones, which are just the reverse in the costatus. The diameter 

 through the umbonial slope is profound. 



2. U. nodiferus. Obtusely subovate, ventricose, moderately 

 thick; surface with a few nodules about the middle of the 

 valves, and smaller ones near the ligament margin ; a slight, 

 not very wide, furrow extends from beak to base; posterior 

 margin approaching to a regular curve; beaks eroded; within 

 white ; cardinal teeth robust, prominent, direct, and profoundly 

 sulcated in old shells; epidermis chestnut-brown. (Length 

 2.1, 1.8; height 1.1, 1; diameter 1.6, 1.4.) 



Approaches U. prasinus, but differs in being proportionally 

 longer, more convex, in having a brown epidermis, narrower 

 anterior side, and oblique posterior margin. 



