>. 
SWALLOW—DESCRIPTION OF FOSSILS. 647 
eurved over the hinge line. Surface marked with about 
twelve simple rounded plications on each side of the mesial 
sinus, and about ten on each side of the mesial fold; and 
with fine concentric imbricating lamelle. 
This fossil resembles the S. opimus of Prof. Hall, but dif- 
fers in the form of the area, the greater number and arrange- 
ment of the plications on the mesial fold and sinus, and the 
much greater number on the sides, and I find no signs of 
longitudinal striz. The number and arrangement of the pli- 
cations, as described above, are constant and unchangeable 
in all the numerous specimens examined. 
Lower Coal Measures of Boone, Randolph and Monroe 
counties, Mo. 
SprrtFER (Cyrtia?) HANNIBALENSIS, 2. Sp. 
Shell large, very short, broad, thick and angular. Ventral 
valve high at the beak, whence it descends abruptly to the 
anterior and lateral margins in lines scarcely arched; sinus 
eep, and very much prolonged into the fold of the opposite 
valve; its bottom descends in a straight line from the be 
to the extreme anterior margin; beak small, pointed, and 
straight; area high, flat, triangular, striated longitudinally 
and transversely ; foramen large. Dorsal,valve nearly as large 
as the ventral; very strongly curved from the hinge line to 
the anterior margin; beak small; mesial fold distinct, eleva-» 
ted and flattened along the center; cardinal line long, 
straight, salient. Surface marked on each side of the mesial 
old and sinus with from fifteen to twenty broad, it 
dinal strie and by small concentric strie and imbricating 
lamella, 
Length, 1.90; breadth, 8.10; thickness, 127. 
This beautiful species is rare in the Lithographic Lime- 
Stone of Marion county. 
CyrtTia MissouRiEnsiIs, D. Sp. 
_Shell small, angular, beak and cardinal extremities salient. 
Ventral valve very prominent, regularly tga 
i a 
orsal valve; area somewhat regularly concave, triangular ; 
the hinge line but little longer than the sides, and rin pe 
tl € greatest width of the shell; foramen Tat er - ; 
bove the 
& Cony ‘ oa : is erforated abo 
eam pseudo-deltidium, which 18 P flattened to- 
