Dr. Hildreth on fresh water Shells. 283 



Shell, narrowed and thin before ; rounded and wider be- 

 hind, beaks slightly elevated ; hinge margin, compressed, 

 carinate ; basal margin, falcated, emarginate and compres- 

 sed ; anterior margin, rounded ; epidermis, dark brown ; 

 surface, rough and scaly ; waved transversely, having dis- 

 tant, irregular, transversely compressed tubercles ; a broad 

 nodulous ridge, extending from the beaks to the anterior 

 basal edge, and projecting on that part ; small oblique waves 

 radiating from the ridge to the hinge and anterior dorsal 

 margins ; cardinal teeth, sulcated ; lateral teeth, striated ; 

 posterior cicatrix, deep and not very rough ; cavity of the 

 beaks, angular, compressed and directed backwards ; naker,, 

 white and moderately iridescent. 



No. 13. — Unio Cvlindricus. Fig. 13. 



Shell, much elongated transversely, sub-cylindrical ; disks, 

 flattened, beaks not much elevated ; teeth, sulcated ob- 

 liquely. 



Length, 1.5 ; breadth, 3.5 ; diameter, 1.2. — Hab. Musk- 

 ingum. 



Shell, thick, and elongated before ; ligament, much de- 

 pressed between the valves ; hinge margin, strait and eleva- 

 ted ; anterior dorsal margin, truncate and emarginate ; pos- 

 terior dorsal, rapidly narrowed; posterior margin, rounded 

 and shortened ; anterior margin, rounded ; anterior basal, 

 projecting ; basal margin, shortened and arcuated ; epider- 

 mis, olivaceous, wrinkled transversely, and maculated with 

 deep green pyramidal spots, with the base inverted between 

 the wrinkles ; a broad nodulous ridge, extending from the 

 beaks to the anterior basal margin, and projecting in front ; 

 with small elevations radiating from the ridge to the hinge 

 and anterior dorsal margins ; cardinal teeth, deeply crena- 

 ted ; lateral teeth, long and well defined ; cavity of the 

 beaks, deep, and directed backwards ; posterior cicatrix, 

 deep and rough ; naker, pearly white, with colored spots ; 

 beautifully iridescent on the fore part. 



Remark. — I have every size of this shell, from one inch to 

 full grown. 



No. 14. Unio Phaseolus. — Fig. 14.* 

 Shell, long ovate, thick ; disks, rather flattened, ligament 

 higher than the beaks ; beaks, depressed and decorticated. 



* White variety of Cuneatus, No. 3. (D. H. B.)— E»- 



