646 TRANS. OF THE ACAD. OF SCIENCE. 
Sprrirer Lirront, n, sp. 
abruptly so towards the lateral margins and beak; sinus well 
defined but shallow, usually much prolonged into the fold of 
the opposite valve, and marked by one depressed plication in 
the middle, and one obsolete fold on each side, which is most 
from eight to ten simple rounded plications on each side of 
the mesial fold and sinus, and with numerous concentric fine 
imbricating lamelle. - 
This shell resembles Sp. Boonensis, but differs in the forms 
and markings of the mesial sinus and fold, and in the length 
of the cardinal line. 
Length, .82; breadth, .83; thickness, .61. ‘ 
Common in the St. Louis Limestone of St. Louis county. 
SprrtFER BooNnensis, n. sp. 
oramen large, higher than wide; hinge line variable, longer, 
shorter or equal to the greatest width of the shell. Dorsa 
