96 
A ug. F . Foerste 
Girardeau limestone. This fauna has a Silurian aspect, and here 
forms the base of the Niagaran section apparently. The fossilif- 
erous layer of limestone here described, and the underlying thin 
bedded layers, are more or less oolitic and form a section about 
33 inches thick a short distance south of the exposures containing ; 
the upper faunal layers here described. There is a line of uncon- 
formity beneath, indicated by a wavy surface of the underlying 
rock, along an irregular contact, marked loo feet south of the main 
fossil locality by a series of nodular masses occurring at success- 
sively higher elevations in the series. 
Rhynchonella (Stegerhynchus) whitii-praecursor, var. nov. 
{Plate III, Figs. 47 A, B, C.) 
Lateral outline broadly ovate, both valves convex, the convexity 
of the brachial valve being slightly greater. The pedicel valve is 
distinctly flattened from the point where the plates supporting 
the teeth terminate on the interior of the shell as far as the point 
where the downward curvature of the shell at the anterior margin Ij 
begins. In one specimen, a cast of the interior, with a length of h 
8 and a width of 9.5 mm., the depth is 7 mm. In most other speci- |i| 
mens, possibly less mature, the depth is nearer 5 or 6 mm. Two | 
plications occupy the median fold of the brachial valve rising | 
scarcely a millimeter above the neighboring plications. A single 
plication occupies the sinus of the pedicel valve. This plication 
equals in size the plications bordering the sinus. i 
Interior of the brachial valve marked by plications where the 
exterior is marked by the grooves between the plications. This 
results in a median elevation in the interior of the brachial valve. 
While the other plications on the interior of the valve become 
indistinct before reaching the crural plates, the median plication 
is strengthened by a thickening of the shell posteriorly and forms 
a median elevation which broadens slightly on reaching the ante- 
rior margin of the crural plates, apparently filling in the space just 
beneath these plates. The crural plates present a slightly concave 
surface approximately parallel to the plane of the valve, and pro- 
ject forward at their inner angles so as to form crural tips. The 
shell beneath the crural plates is thickened and filled out so that 
only a narrow space is left between these plates, and this space is 
occupied by a very narrow, linear septum, representing the car- j 
dinal process. 
