310 _ The American Geologist. eae 
Fics. 20 and 21. Front edge and cardinal line of the united valves, 
x. 
Fic 22. Lateral view of the shell showing the strong elevation of the 
mesial fold, x. 
This splendid Spirifer is not likely to be confounded with 
any other form of the same horizon. It is broader than iong 
and, without the mesial fold, would be semicircular in outline. 
The cardinal extremities are rounded and the cardinal area 
is narrow and with nearly parallel edges. There are from 
twenty to twenty-five low, rounded plications on either side 
of the mesial fold. The plications on both valves are dichot- 
omous. The elevation of the mesial fold at the front edge 
of the brachial valve is extravagant and the plications that 
traverse the fold are broad and with little or no elevation. 
The pedicel valve has from twenty to twenty-five low, 
rounded, dichotomizing plications either side of the broad, 
deep sinus. There are from twelve to fifteen flattened plica- 
tions in the sinus, the ones on the sides being broadest. The 
beak of the pedicel valve is but little incurved and is separated 
from the beak of the brachial valve by half the width of the 
cardinal area. The aperture is uncovered and a low, broad 
triangle. 
Both the mesial fold and sinus begin near the ends of the 
beaks. Two strong lines of growth cross the plications and 
finer undulating lines traverse the front of the shell. The test 
is rather thin and the diameter from valve to valve is little, 
compared with the otherwise great dimensions of the shell. ° 
The semicircular outline, great elevation of the mesial fold, 
great depth of the sinus, general depressed character of the 
shell at the umbonal region together with the narrow cardin- 
al area and the round dichotomizing plications will serve to 
identify this spirifer. In size it is hardly less than S. grimesi. 
It comes from the fifth and sixth horizons of the Lower Burlington 
and the basal layers of the Upper Burlington limestone at Louisiana, 
Mo. 
The type, as well as all other fossils described in this paper, be- 
_ longs to the author’s collection. 
All figures on the plate are of natural size, except 18, 19, 20, 21 
22, which are reduced (X14). 
Feb. 20, 1902. 
i ee ek ee ee i 
