110 PALEONTOLOGY OF THE EUREKA DISTRICT. 
muscular scars. The posterior adductor scars (pa) are large, elongate-oval 
in outline, and so situated that their anterior portions cross a line drawn 
through the transverse center of the valve, while the posterior ends point 
back to their path of advance; they are separated by the track of advance 
of the anterior adductor scars (aa), which are seen just in advance of 
them as very small, slightly impressed oval spaces. The area of the ad- 
justor muscular scars (ad) and their path of advance is, as a whole, more 
or less triangular, and situated one on each postero-lateral portion of the 
valve. The front margin of the area is rather broad and divided into four 
small, somewhat indistinct lobes; posteriorly the area contracts to the in- 
itial point of the path of advance; two scars appear to be defined on the 
anterior two-thirds of each area; they are elongate and include two of the 
little lobes of the anterior margin within the area of each, and appear to 
represent the point of attachment of the exterior and posterior adjustor 
muscles; there is a small space divided off by raised lines that may be the 
central adjustor scar, but it is not clearly defined. 
Between the areas of the adjustor scars and the large central area of 
the adductors there is a sharp, narrow ridge that distinctly separates them. 
A narrow elongate area (pp), outside of the area of the adjustor scars, was 
probably the point of attachment of the posterior parietal muscle, and the 
lateral and outer walls of the perivisceral cavity seem to have left evidence 
of their presence on the margins (w) of the central area. 
The great pallial sinuses approximate quite closely towards the front, 
curving gently outward and backward around the central area to opposite 
the anterior margin of the area of the adjustor muscular scars, around 
which they curve to the narrow margin just within the exterior border of 
the shell; their further course cannot be traced, but, from the narrow area 
between the posterior parietal area and the margin, the sinuses must have 
been very narrow, and with very minute, if any, lateral or internal ramus- 
cules. The anterior lateral ramifications of the sinuses are strongly defined 
as they spring from the main sinus, becoming smaller and bifurcating or 
branching towards the margin of the valve; no inner ramifications are to 
be seen on the space (7) between the great sinus and the perivisceral area 
on the dorsal valve, a feature, however, that is well shown on a fragment 
