Paleontology Zorritos Formation' 119 



in cross-section ; the growth here noted could hardly have 

 been so shaped even if weathering away were offered as a 

 cause for its possible reduction ; the comparatively clear-cut 

 condition of all similar parts of the shell suggests strongly 

 the impossibility of such a change. 



A. scptifcra belongs in the group of similar forms in the 

 section Anadara of which A. grandis and A. camuloensis are 

 members. The former of the two is living at present on the 

 west coast ; the other occurs in the Miocene and later of 

 California. A. septifera differs from A. camuloensis 

 Osmont "" in being less quadrate, more rounded, with fuller, 

 less pointed beaks, the hinge-line proportionally longer, and 

 the rib-sculpture less regular than in camuloensis. 



Upper Zorritos. Quebrada del Grillo. 



Area (Anadara) nelsoni n. sp. 



Plate VI— Figs. 7, 8. 



Shell thick, stout, of moderate size, rounded in contour. 

 Anterior margin broadly rounded, meeting hinge-line at an 

 angle slightly more than 90° ; ventral margin arcuate, pos- 

 terior margin recurving into a straight line which meets the 

 hinge at an angle of from 135° to 145°. Umbones dome-like 

 viewed laterally, high, inflated, prosog^rate. Umbonal slope 

 smoothly convex anteriorly and mesially, with slight excava- 

 tion near the anterior margin in some individuals ; a poste- 

 rior keel runs from the marginal angle to the umbone. Pos- 

 terior surface slightly excavated. Ribs, 30-31 in number, 

 prominent, square in cross-section, smooth posteriorly and 

 somewhat irregularly noded anteriorly by concentric chan- 

 nellings which cross the interspaces but are not as prominent 

 there. Interspaces deep, narrower than the ribs in unweath- 

 ered specimens. Area short, bounded by a deep marginal 

 furrow and an elevated band, and bearing about three wave- 



Univ. Cal. Bull. Geo!., vol. 4. p. 98, pi. 10, figs. 6, 6a, 1904. 



