Fauna of the Morrow Group 
199 
of the valve, and perforated by a large subcircular foramen; 
delthyrium concealed. 
Bracial valve faintly trilobate, nearly or quite as convex as 
the pedicle, greatest convexity posterior to the middle; mesial 
fold originating near the umbo but not becoming defined until 
anterior to the middle, rounded or slightly flattened on top, 
rarely marked by a faint median groove toward the anterior 
margin, but more commonly with its median line defined by an 
indenture of the lines of growth along its middle; on each side 
of the fold there is a faint, ill-defined, rounded sinus origi- 
nating in front of the middle and becoming more prominent 
toward the margins ; beak strongly incurved beneath that of the 
opposite valve. 
The surface of both valves is marked by close-set, concentric 
lines of growth with the fourth or fifth lines stronger than 
the intermediate ones when the preservation is such as to show 
the latter; the stronger growth lines are sometimes crowded 
toward the front; ordinarily the shell is otherwise smooth but 
in some of the specimens faint radiating striae may be observed 
running from beak to margin. 
Remarks, Like all other species of Composita this is an ex- 
ceedingly variable one and embraces individuals which closely 
resemble certain forms ordinarily identified as C. trinuclea, C. 
subquadrata, or C. subtilita. Typical forms embodying what 
may be called the average characters of the species are, how- 
ever, notably different from the typical forms of those species. 
C. ozarkana differs from C. trinuclea in its larger size, its greater 
breadth in proportion to its length, and the generally fainter 
and more poorly defined character of its fold and sinus. From 
C. subquadrata it may be differentiated by the presence of the 
sulcus in the sinus of the pedicle valve and by its less quadrate 
outline. In comparison with the plesiotypes of C. subtilita, C. 
ozarkana is characterized by the greater transverse convexity 
of the pedicle valve, the slightly anterior extension of the mesial 
sinus, the weakness of the lateral sinuses bordering on the fold 
of the brachial valve, and the fact that the greatest width of 
the shell is near the middle point instead of notably anterior 
to it as in the Coal Measures form. 
