334 CALIFORNIA ACADEMY OF SCIENCES. 



Pholadomya it may be proved by better specimens to be 

 allied to Carditaox Petricola, both having species of sim- 

 ilar outline and sculpture, but of course very different 

 otherwise. No sign of their strongly toothed hinge is 

 visible in the specimens yet seen. 



6. Pholadomya (Triplicosta) progressiva n. sp. Plate 

 xlviii, figs, ii and 12, natural size. 



Outline oblong quadrilateral ; length varying from nearly 

 twice the height to a little less than height; cardinal area 

 posteriorly well-defined ovate, large; valves gaping mod- 

 erately behind; umbos moderate, at about |th of length 

 from anterior end and a quarter of an inch apart; ribs 

 nodular from irregular lines of growth, the posterior 

 broad ones covering from half to two-thirds of surface, 

 and about 10 in number, then each gradually dividing into 

 3 01-4 narrow sharp ones; the posterior 15 to 18, covering 

 the rest of surface, with intervals equal in width to ribs. 

 The shell seems to have been about one-eighth of an inch 

 thick, and the inside had corresponding rib-like markings. 

 Length of largest 2.75 inch, height 2.50, breadth 2.00. 



Found by W. L. Watts, along Santa Paula and Sespi 

 Creeks, branches of the Santa Clara River, Ventura 

 Co., and by another collector near San Luis Rey, 

 San Diego Co., associated with Cardita -planicosta and 

 other Cret. B (or Eocene) fossils. Four specimens are 

 from the former and three from latter locality. They 

 vary considerably in outline from both counties, from 

 pressure. 



All the 200 or more species of Pholadomya described 

 seem very variable, and many of them should probably 

 be called varieties, as a study of about half the figures 

 published indicates. None of them, however, has the 

 triplicate ribs of this species. 



In reviewing the West Coast species some corrections 



