178 PALAEONTOLOGY OF CALIFORNIA. 



PHOLADOMYA, Sby. 

 P. OREGONENSIS, n. B. 



PL 29, Fig. 65. 



SHELL small; beaks large, prominent, anterior; anterior end 

 sloping outwards and downwards from the beaks, nearly straight 

 above, narrowly rounded below; posterior end prominently 

 rounded, a little the most prominent above the middle ; cardinal 

 margin nearly straight behind the beaks; base broadly and 

 regularly convex. Surface marked by small but pretty regular 

 concentric lines, crossed by a few faint radiating ridges, three or 

 four of which, on the middle and anterior end, are distinct, the 

 others obsolete. 



Length, 1.2 inch ; width, 7 inch. 



Rare in the Chico Group, near the Toll House at the summit of the Siskiyou 

 Mountains, near the southern boundary of Oregon. 



From P. nasuta, this shell can be distinguished by its flatter form, the absence 

 of the anterior truncation, its regularly rounded posterior end, and its obsolete 

 radiating ribs. There is no other species on the West Coast, with which it could 

 be confounded. P. subelongata, Meek, from Vancouver Island, has many more 

 ribs, and they are most marked on the posterior portions of the surface. 



PLEUROMYA, Agas. 



P. PAPYRACEA, n. S. 

 PI. 29, Fig. 66. 



SHELL very thin, beaks prominent, and placed about a third 

 of the length from the anterior end ; cardinal margin sloping 

 slightly, and nearly straight posteriorly, anterior end excavated 

 under the beaks, and abruptly truncated below, very slightly 

 gaping; posterior end narrowly rounded, gaping more than the 

 anterior ; base broadly convex. Surface ornamented by irregular 

 concentric ribs. Posterior muscular scar small; pallial sinus 

 broad and deep. 



