70 PALEONTOLOGY OF THE EUREKA DISTRICT. 
on its ventral valve with relation to the apex of the valve and in the absence 
of any indication of a false area. The general facies of the shell also suggests 
another generic group. Its affinity with Acrotreta consists in the presence 
of the three pairs of muscular scars in the smaller valves and in the perforate 
larger valve. The surface structure and general form closely ally it to 
Siphonotreta, and I would refer Siphonotreta fissa Kutorga, to this genus, 
as in all external characters it is almost specifically related to Schizambon 
typicalis. 
Schizambon typicalis, n. sp. 
Plate i, figs. 3a-d. 
Shell small, longitudinally ovate; length and breadth nearly the same; 
front and side margins broadly rounded; posterior margin of the ventral 
valve slightly acuminate, of the dorsal valve broadly rounded, except a 
very slight projection at the center. The ventral valve has a small, oblong 
foramen near the apex, with a gradually narrowing depression extending 
to the posterior extremity. Both valves are slightly convex, the ventral 
one the more so. Surface marked by lamellose concentric raised lines, 
bordered with fine spines on the outer portions of the shell. Towards the 
beak numerous fine concentric strize appear. 
The shells consist of an outer nacreous layer, with an exceedingly thin 
inner calcareous layer closely attached to it. As has been mentioned under 
the generic description, the shell appears to be punctate. In many instances 
scarcely a pit is seen on the inside of the shell, while in others they are 
scattered quite thickly over the surface. The largest specimens obtained 
measured 5™™ in length by 4.5™™ in breadth. 
In the ventral valve, fig. 3, the margin of the siphonal opening is 
slightly elevated and extends quite to the beak, leaving a triangular, elon- 
gate, narrow depression behind the opening. Starting near the beak and 
running obliquely outward a slightly-depressed, narrow, smooth line shows 
the advance of a muscular scar; between this and the siphonal opening a 
smooth space intervenes, in the front of which a pair of muscular scars can 
be detected. A slight depression extends forward in advance of the siphonal 
opening. 
