Description of New Species of Fossils. 227 



Leptobolus occidentalis n. sp. 

 Plate 7, fig. 18. 

 Shell minute, usually measuring about eight-hunclredths of an inch 

 in length in full grown individuals ; orbicular or broadly ovate in 

 outline, widest below the middle, and somewhat pointed at tliebeak; 

 valves moderately convex, most prominent near the beak. 



Surface marked by concentric lines of growth sometimes forming 

 varices. 



Formation and locality. Very numerous in some dark shales of 

 the Hudson-river group (probably of the Utica slate liorizon) at 

 Hawley's mill, twelve miles west of Dubuque, Iowa; and at Platt- 

 ville, Wisconsin, on the authority of specimens labeled by Mr. T. J. 

 Hale. 



Leptobolus insignis n. sp. 

 Plate 7, fig. 17. 

 Shell minute, orbicular, with a scarcely pointed beak; valves regu- 

 larly convex when not compressed. Specimens usually flattened. 

 Surface marked by concentric lines of growth. 



This species differs from the preceding in being more nearly circu- 

 lar, with a shorter beak and more regularly convex valves, and both 

 this and the preceding species differ from L. lepis in being much 

 less elongate, and not so distinctly oval. 



Formation and locality. • In the Utica slate at Middleville in Her- 

 kimer county ; near Fort Plain ; at Utica, and other places in N^ew 

 York. 



Genus— LYEODESMA Conrad. 

 Annual Geol. Report, N. Y., for 1841, p. 51. 

 " Equivalved, inequilateral; hinge with about eight diverging pro- 

 minent cardinal teeth, transversely striated." 



The above description of Mr. Conrad has been verified in the dis- 

 covery of additional specimens of the L. "plana Conr. and of other 

 species of the genus. The following species possess but six diverg- 

 ing crenulate teeth, showing the necessity of modifying the generic 

 description. 



Ltrodesma Cincinnatiensis n. sp. 



Plate 7, fig. 28. 



Shell small, subrhomboidal in outline, and obtusely pointed at the 



postero-basal angle ; -valves moderately convex with a subangular 



umbonal ridge and narrow cardinal slope ; anterior end rounded and 



passing into the more broadly rounded basal line ; posterior end 



