176 PALAEONTOLOGY OF CALIFORNIA. 



distinguish it. The umbonal ridge is not an exsert angle, but a mere dislocation 

 of the surface ; as if the shell had been broken along that line, and then set back 

 a short distance. 



L. SUBCIRCULARIS, 11. S. 

 PL 24, Fig. 160. 



Shell small, subcircular, compressed; beaks small, central, 

 pointing anteriorly ; cardinal margin slightly excavated in front 

 of the beaks, straight, acute, and sloping gently downwards pos- 

 teriorly; anterior and basal margins forming a nearly perfect 

 semicircle ; posterior end faintly subtruncated ; a faint umbo- 

 nal ridge, behind which the surface is gently concave. Surface 

 marked by small, regular, concentric ribs, which become nearly 

 obsolete posterior to the umbonal ridge ; these ribs are acute 

 and sublamellar near the beaks, and rounded on the lower por- 

 tion of the shell. 



Figure, slightly magnified. 



Locality: Collected with the preceding. 



L. CUMULATA, U. S. 

 PL 24, Fig. 254. 



Suell minute, subcircular, thick; beaks large, subcentral; 

 ends and base regularly rounded; anterior end slightly emar- 

 ginate immediately under the beaks ; cardinal margin nearly 

 straight, uniting with the posterior end by a rounded angle. 

 Surface marked by four or more enormous, rounded, concentric 

 ribs, giving the shell the appearance of being composed of a num- 

 ber of independent masses laid one over another ; besides these 

 ribs there are a few small, oblique, divaricating, impressed lines, 

 most marked near the apex. 



Figures, about two and a half times the natural size. 



Locality: Near Fort Tejon; very rare. The figure and description are from a 

 single specimen. 



I have seen but one fragment of another from the same locality. 



