25 



inclosing an angle okbout 80°, and extending a little more than 

 one-third the length) f the shell ; sides sub-parallel for a short 

 distance; front rouled; valves very slightly convex. Sur- 

 face marked by ratlr strong concentric lines of growth, and 

 a few striae on partlly exfoliated specimens. 



This species varii from the last in being five or six times as 

 large, measuring ntrly three-fourths of an inch in length by 

 a little more than Hf an in^h in width. The form is distinct 

 from every other spies described from rocks of the same age. 



Geological Formcon and Locality. In rocks of the age of 

 the Potsdam sandstie, Trempeleau, Wisconsin. James HaU. 



SIRIFER gibbosus, (n. s.) 



Description. Shesomewhat below the medium size, gibbous, 

 beaks slightly moved, area less than the width of the shell 

 below ; about thredmes as wide as high ; cardinal extremities 

 rounded ; foramen rge, higher than wide ; valves marked by 

 about four simple rinded plications on each side of the mesial 

 elevation. Dorsal live extremely gibbous on the umbo, regu- 

 larly arcuate trans vsely ; mesial elevation broad, flattened on 

 top; plications noextending to the beak, which is a little 

 incurved. Ventra^alve deeper than the dorsal, but less 

 arcuate; mesial depssion broad and deep, rounded at bottom. 

 Surface of both vaes (in well preserved specimens) show evi- 

 dence of fine radiate striae. 



This species resales S. crispus of the Niagara group in 

 New York, but is r!ch larger, frequently more than twice as 

 wide as that specie the mesial elevation is wider and not so 

 high. The specims described are all internal casts, so that 

 the external surfa()characters cannot be fully given. 



Geological Formon and Locality. In limestone of the age 

 of the Niagara g4p, Racine, Wisconsin. Dr. P. R. Hoy, 

 T. J. Hale, James tail. 



SIRIFER eudora, (n. s.) 



Description. Sh 1 of moderate size, transversely sub-ovate, 

 length and width fthreo to four, valves extremely gibbous; 

 hinge line less thl the width of the shell below ; cardinal 

 extremities roundeirea moderately high ; foramen triangular, 

 a little higher thavide. Valves marked by three to four 

 simple, strong, angur plications on each side of the mesial fold 

 and sinus. Dorsal (ve regularly arcuate ; beak somewhat in- 

 curved; mesial folof moderate width, flattened above and 

 slightly depressed iihe lower part, Ventral valve most prom- 

 inent near the umb< back strongly incurved over the area; 



