904 Report oF THE State GrOLoGIst. 
cardinal line may be regarded as extending nearly to the lateral 
extremities of the valves; the articulating apparatus consisting 
of a pair of long marginal ridge-like teeth on the divergent 
cardinal slopes, fitting into narrow marginal grooves on the 
brachial valve. There is sometimes a trace of a median septum 
over the pallial region. In the brachial valve is a small callus, 
boss or cardinal process lying directly beneath the apex. Below 
this is a strong median septum, which increases in height ante- 
riorly and rises to an acute, anteriorly directed apex at about two- 
thirds the length of the shell. In front of this point its anterior 
edge is concave, the septum disappearing not far within the 
margin of the valve. 
No traces of muscular scars have been observed on either 
valve. 
The external surface of the valves is covered by a coarse net- 
work of superficial cells, usually hexagonal, sometimes circular in 
outline. In all species and in early growth-stages there is a bare, 
smooth triangular area at the beak of the pedicle-valve, where 
this superficial ornament does not extend. 
Type, Hichwaldia subtrigonalis, Billings. 
Distribution. Lower Silurian — Lower Devonian. 
Aulacorhynchus, Dittmar. 1871. 
(Plate 54, figs. 23, 24.) 
Shells short, transversely elongate or alate; extremities often 
rounded ; hinge-line straight, usually making the greatest width 
of the shell. Valves very thin and fragile. Pedicle-valve slightly 
convex, with traces of a broad, obscure median sinus; brachial 
valve flat. Surface covered with numerous regular and continu- 
ous, concentric rounded folds or ridges which are separated by 
furrows of equal width. 
In the pedicle-valve the character of the articulating processes 
has not been fully ascertained. There appears, however, to have 
been no cardinal area, and but exceedingly small teeth, judging 
from the structure of the brachial valve. Just within the apex of 
the valve, which is closely appressed against the opposite one, 
begins a pair of divergent, elevated ridges, which extend for one- 
third, or even one-half the length of the shell, and inclose a 
thickened area or platform, which terminates abruptly in a trans- 
verse anterior margin. This platform is the seat of the adductor 
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