ARNOLD] FOSSIL MOLLUSKS FROM CALIFORNIA 429 



periphery ; minute sculpture as in right valve ; ears flat, the anterior 

 one showing two or three faint radial riblets. Interior of both valves 

 reflecting the external ribbing very prominently. 



Dimensions. — Longitude, 70 mm. ; altitude, 64 mm. ; diameter 

 (approximate), 12 mm. 



Notes. — This species appears to be most closely related to P. sanc- 

 tcecruzensis Arnold, which occurs in the Oligocene and lowest Mio- 

 cene of the Santa Cruz Mountains. It is distinguishable from the 

 latter by its larger size, flatter disks, less elevated ribs, and by the 

 prominent reflection of the external ribbing on the interior of the 

 disk. This last is one of the most prominent, unique, and interesting 

 characteristics of P. vanvlecki. Named in honor of Mr. Robert Van 

 Vleck Anderson, of the United States Geological Survey. 



Type.— Cat. No. 165,305, U. S. N. M. (right valve). 



Paratype. — Cat. No. 165,306, U. S. N. M. (left valve). 



Locality. — Mouth of Ballard Canyon, 2 miles south of Santa 

 Ynez, Santa Barbara County, California ; locality No. 4478. 



Horizon. — Vaqueros formation, lower Miocene. 



MODIOLUS YNEZIANUS, new species 

 PI. LII, %. 2 



Description. — Shell averaging about 60 millimeters in altitude, 

 elongate-ovate in outline, convex, equivalve; beaks nearly terminal, 

 protruding forward and slightly beyond margin ; base not regularly 

 rounded, curving sharper anteriorly; anterior margin curving sharply 

 around attenuate extremity just in front of beak, and then straight 

 for nearly entire length of shell, with the exception of a slight con- 

 traction near middle caused by a sulcation extending obliquely back- 

 ward from beaks ; posterior dorsal margin straight, bending around 

 a moderately angular extremity into the slightly arcuate ventral mar- 

 gin ; the shell bulges in the middle in such a way as to suggest a 

 broad, rounded ridge bowing over obliquely backward from the an- 

 terior part of the base to the beak ; surface sculpture consists of 

 fine incremental lines. 



Dimensions. — Of type, a small specimen; altitude, 31 mm.; lati- 

 tude, 18 mm.; diameter, 11 mm. 



Notes. — This species is allied to M. fomicatus Carpenter, from 

 which it differs by being much larger, more angular posteriorly, 

 not as ventricose nor with the bulging part as overturned posteriorly. 

 It is quite abundant at the type locality and at other places where the 

 Vaqueros formation is fossiliferous. Named for the Santa Ynez 

 Mountains. 



