14 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Anterior valve with eight radiating series of strong, regu- 

 larly increasing, closely ranked, rather smoothly rounded 

 tubercles, 7-8 in a series; posterior edge of valve dentated 

 by a pair of similar series of larger, more compressed and 

 backwardly directed tubercles; entire surface between these 

 rib-like series rather finely sculptured by a quite uniform 

 reticulum of pitting and netting arranged almost in a checker 

 pattern; all sculpture worn or obsolete toward apex. 



Median valves with lateral areas strongly elevated; general 

 surface of these sculptured like the anterior valve; bounded 

 behind by a sutural series of 9-10 strong, oblique tubercles 

 dentating the suture, and in front by a much wider, stronger 

 diagonal rib, formed by the partial coalescence of its com- 

 ponent, strongly transversely elongate, compressed tubercles, 

 and often more or less bifurcated by a shallow furrow down 

 its center. Central areas very neatly sculptured by strong, 

 inwardly arching, longitudinal riblets, traversed by slightly 

 more zigzag, and otherwise rather less regular, obliquely 

 arcuate riblets radiating from the beak and ridge, the whole 

 forming a very regular net-like reticulum, the meshes of 

 which are represented by fairly deep, squarish pits, always 

 larger and deeper toward the sides; the posteriormost of 

 the radial threads characteristically stronger than any of 

 those in front, and the pits enclosed between it and the 

 diagonal rib larger and deeper than those in front of it, thus 

 sharply setting off the diagonal rib and lateral areas from 

 the central areas; jugal area in the posterior two or three 

 valves with a very narrow smooth tract on the ridge, the 

 netted sculpturing in the other valves extending clear over, 

 merely becoming a little less regular on the ridge where the 

 central areas come together. 



Posterior valve with the mucro at about the posterior 

 third of the tegmentum, scarcely elevated; diagonal ribs 

 weaker than those of the intermediate valves; area in front 

 sculptured similarly to the central areas of the intermediate 

 valves, but with fewer ribs on a side, and although the pits 

 in front of the diagonal ribs are a little larger than the 

 remainder, the difference is less conspicuous here than in 

 the other valves; posterior area sculptured with irregularly 

 nodulose, longitudinal ribs, corresponding in position to the 



