no. 1823. MOLLUSKS OF THE GENUS CERITHIOPSIS—BARTSCH. 361 



terior edge of the peripheral cord. Periphery marked by a strong 

 spiral cord which is separated from the supraperipheral cord by a 

 sulcus as wide as that which separates the supraperipheral cord from 

 the median and, like it, crossed by the extensions of the axial ribs, 

 which terminate at the posterior edge of the peripheral cord. Base 

 slightly concave, with two strong spiral cords. Aperture broadly, 

 irregularly oval, decidedly channeled anteriorly; posterior angle 

 obtuse; columella somewhat curved and twisted, with the free edge 

 reflected. 



Doctor Arnold's type, which came from the Pleistocene of San 

 Pedro, was lost in transit to the U. S. National Museum, had seven 

 post-nuclear whorls and measured: Length 2.5 mm., diameter 1 mm. 



Named for Mrs. M. Burton Williamson, of Los Angeles, California. 



CERITHIOPSIS TRUNCATA Dall. 



Plate 40, fig. 2. 



Cerithiopsis stejnegeri truncata Dall, Proe. U. S. Nat. Mus., vol. 9, 1886, p. 304, 

 pi. 4, fig. 5. 



Shell wax yellow. (Nucleus and early whorls decollated.) The 

 four remaining well rounded, ornamented with three tuberculate spiral 

 keels of which the median is the strongest; the weakest is at the sum- 

 mit, which it crenulates. The last is midway between the suture and 

 the median cord. In addition to the spiral cords, the whorls are 

 marked by low, well rounded, vertical, axial ribs, the intersection of 

 which with the spiral cord renders them feebly tuberculate. Of these 

 ribs, 30 occur upon the penultimate whorl and 22 upon the preceding 

 turns. The spaces inclosed between the spiral cords and the axial 

 ribs are squarish, well impressed pits. A slender thread of the base 

 is apparent in the somewhat constricted suture. Periphery of the 

 last whorl marked by a sulcus which is about as wide as the spaces 

 between the first and median cord and is crossed by feeble continu- 

 ations of the axial ribs. Base short, well rounded, with a slender 

 fasciole at the insertion of the columella; entire surface of spire and 

 base crossed by rather strong, incremental lines and numerous fine 

 spiral striations; aperture broadly ovate; decidedly channeled anteri- 

 orly ; posterior angle obtuse; outer lip rendered wavy by the external 

 sculpture; columella short, very broad, curved; parietal wall glazed 

 with a thin callus which extends from the columella to the posterior 

 angle. 



The type (Cat. No. 213303, U.S.N.M.) and two specimens were 

 collected in the canals of sponges at Unalaska. The type measures: 

 Length 3.1 mm., diameter 1.9 mm. Nine specimens (Cat. No. 

 213304, U.S.N.M.) collected at the same locality under similar 

 conditions. Doctor Dall noted when he collected the specimens that 



