200 PALAEONTOLOGY OF CALIFORNIA. 



from the beaks to near the middle of the shell, and ending 

 abruptly. 



Figures, natural size. 



From a buff-colored shale east of New Idria, at or very near the summit of the 

 Tejon Group, where I found it very abundant. 



P. operculiformis and P. Californicus of the California Cretaceous are marked by 

 concentric lines only ; the former is much larger than the present species, and the 

 ears are of a very different shape ; neither of those species has the internal ribs of 

 this shell. The other species are longer, with smaller ears, and are markedly 

 costate. 



NEITHEA, Drouet. 



1ST. GRANDICOSTA, 11. S. 

 PL 33, Fig. 99 a. 



Shell minute, very inequivalve, equilateral, elongated ; lower 

 valve, sides tapering a little concavely ; general contour of the 

 base semicircular, with six prominent angles and concave inter- 

 spaces. Surface marked by six very large, round, equidistant 

 ribs, the interspaces carrying a small rib, flanked on each side by 

 one still smaller; in some cases the sides of the large ribs are 

 faintly striate ; crossing these ribs are minute, very regular lines 

 of growth; ears very unequal, the right ear long and deeply 

 emarginate. 



Length from beak to base, .5 inch. 



Eare in the Shasta Group, Cottonwood Creek, Shasta County. 



This is one of the smallest, if not absolutely the smallest species yet described 

 in the genus. It has some resemblance to N. duplicicosta, Roem. sp., of Texas, 

 which has three intermediate ribs ; but in that shell the large rib is very com- 

 pound. It is most nearly allied to the French Neocomien species, N. atava, d'Orb. 

 sp., but the ears are of a different shape ; in that shell the right ear of the lower 

 valve is much smaller, the interspaces are more concave, and the large ribs, instead 

 of being simple, are made up of three smaller ribs. 



