70 PALEONTOLOGY OF THE EUEEKA 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 striae 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 mm in length by 4.5 mm 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. 



