THE NAUTILUS. 69 



Notes. Characterized by definite number and character of 

 ribs, and easily distinguished from P. operculiformis, P. calijor- 

 nicus, P. complexcosia, and P. interradiatus. Named in honor of 

 W. 0. Clark. 



Type. Museum, Stanford University, Calif. 



Locality. Found by W. 0. Clark on the San Jose Quadrangle, 

 two and a half miles Northeast of Milpitos, just at beginning of 

 foothills and a little north of serpentine outcrop. 



Horizon. Horsetown, middle Cretaceous. 



AvicuLA GREGORYi, n. s. Hall and Ambrose. 



Description. Shell oblique, subcompressed ; ears very un- 

 equal ; beaks moderate, anterior; cardinal line straight ; anterior 

 ear short and angular with small byssal notch ; posterior ear 

 broad ; anterior margin regularly rounded from near umbo to 

 base, posterior margin straight and sharply rounded to base, 

 basal margin forms an excentric curve, most produced behind. 

 Ears gradually round into body of shell. Surface marked by 

 numerous faint, but distinct, radiating ribs. 



Dimensions. Altitude 7 mm. ; longitude 9 mm. ; hinge line 

 5 mm.; diameter. 



Notes. Distinguished from A. pellucida by ribs, byssal notch 

 and lack of acuminate posterior ear. Named in honor of A. E. 

 Gregory, Stanford University, Calif. 



Type. Museum, Stanford University, Calif. 



Locality. One and one-half miles S. 10 W. of Carnegie, Tesla 

 Quadrangle. 



Horizon. Horsetown, middle Cretaceous. 



SoNNERATiA ROGERSi, u. s. Hall and Ambrose. 



Description. Shell small, not often above a diameter of 4.5 

 mm.; discoidal, laterally compressed and flattened; umbilicus 

 not large, about one-quarter total diameter, funnel form due to 

 sloping sides and gradual increasing thickness of shell ; sides 

 converge gently toward periphery; ventral surface subquadrate; 

 surface ornamented with about sixty transverse flexuous ribs 

 which usually cross the ventral surface and terminate in about 

 one-quarter as many distinct tubercules upon the shoulder of 



