648 TRANS. OF THE ACAD. OF SCIENCE. 
wards the cardinal extremities, which are somewhat produ- 
ced; mesial fold convex, well defined and bounded by 
grooves larger and deeper than those separating the plica- 
tion; beak incurved against the lower extremity of the del- 
side of the mesial sinus. There is also one plication, some- 
what smaller than the others, on each slope of the mesial 
fold and sinus. The plications bounding the mesial sinus are 
larger than the others; the internal casts of the shell show 
all the plications less distinctly. The shell is also marked 
With fine imbricating concentric lamelle and numerous small 
punctures. 
This species is more nearly allied to the Cyrtia umbonata 
of Prof. Hall than to any other. It differs however from 
that in having plications on the mesial fold and in the sinus 
of the ventral valve ; the plications are more numerous and 
extend to the beak; the foramen is wider at the base. 
Found in the Hamilton Group of Callaway county, Mo. 
where it appears to be confined to a stratum of bluish brown 
it is very abundant, and, so far as observed, is accompanied 
by but two other species, the Atrypa reticularis and Spiri- 
gera Fultonensis. 
See Cyrtra OccIDENTALISs, n. sp. 
Shell of medium size, semi-conical. Ventral valve high, 
semi-conical, slightly curved from the beak to the anterior 
and lateral margins; sinus well defined, very wide and flat- 
and convex in front; cardinal line shorter than the greatest 
width of the shell. Surface marked with some fifteen or 
often become very conspicuous towards the margins; shell 
punctate. 
This species somewhat resembles the Spirifer aspera of 
Prof. Hall; but the mesial fold and the sinus are much wider 
In front, the cardinal extremities much less salient, the area 
a 2 and the shell punctate. 
ery rare in the Hamilton Group of Callaway county, Mo, 
