- SWALLOW—DESCRIPTION OF FOSSILS. 649 
associated. with Atrypa reticularis and Strophodonta de- 
missa. 
SPIRIGERA MINIMA, N. Sp. 
Shell minute, varying from ovate to orbicular, depressed. 
Ventral valve rather convex, slightly flattened towards the 
margins; beak small, incurved; perforation circular, small ; 
mesial sinus broad and subangular. Dorsal valve less convex, 
highest near the beak; beak small, curved beneath the perfo- 
ration of the opposite beak; mesial fold broad and but slight- 
ly elevated. Surface marked with numerous very thin con- 
centric lamella, which stand out nearly perpendicular to the 
surface of the shell; structure punctate; spiral appendage 
very large. 
Length, .28; breadth, .28; thickness, .15. 
This beautiful little species is very rare in the Hamilton 
rocks of Callaway county. 
SPrRIGERA PRovurTIt, n. sp. 
Ventral valve very convex, highest near the beak, whence it 
is strongly arched to the anterior and lateral margins, and 
very abruptly to the cardinal ; beak prominent, incurved, and 
wards the beak. Surface marked with concentric bands and 
ornamented with fine concentric and longitudinal stri@, which 
Sive the shell a fine granulated appearance. 
Length, .66; breadth, .71; thickness, 45. s 
From the Chemung rocks of St. Louis county, near the 
SrrriGeRA HANNIBALENSIS, 0. §P- 
orbicular to transversely 
elle, Ventral 
what flattened 
185 ion circular, usually 
; sinus most 
Valve convex, hi 
