32 PALAEONTOLOGY OF CALIFORNIA. 



This genus bears almost the same external relation to Avicula, on one side, that 

 Monotis does on the other. None of the specimens show more of the hinge than 

 that it is straight and slightly thickened. 



R. OBESUS, n. s. 



PL 5, Fig. 30, a, b. 



SHELL oblique, very convex. Umbones prominent, placed at 

 the centre of the cardinal line. Hinge straight, ear acuminate, 

 small ; posterior margin broadly and regularly convex. Anterior 

 margin sinuous. Surface marked by irregular lines of growth, 

 and occasionally by obsolete concentric undulations. 



Figures, natural size. 



Locality; Rattlesnake Point, near Humboldt City, Nevada Territory. 



POSIDONOMYA, Bronn. 



P. STELLA, n. S. 

 PI. 6, Fig. 81. 



SHELL subcircular, convex, inequilateral; beaks prominent, 

 anterior. Cardinal margin rounded in advance, straight and 

 slightly sloping posteriorly, and uniting with the anal margin by 

 a curve. Surface smooth, or marked by a few regular, concentric, 

 rounded ribs. 



Length, .9 inch; width, .75 inch; depth of valve, .18 inch. 



Locality: Star CaSon, Humboldt Mining Eegion. 



P. DAYTONENSIS, n. s. 



PI. 6, Fig. 32. 



SHELL flattened, oblique ; beak small, not prominent. Cardinal 

 line straight, shorter than the greatest length of the shell. Buccal 

 margin excavated below the hinge-line, convex below, merging 

 insensibly into the base. Anal margin convex below, nearly 

 straight above, and inclined inwards to meet the hinge-line. Sur 

 face depressed, marked on the cast by a few faint concentric un- 



