AKNOLD — THE PALEONTOLOGY AND STHATIGBAPHY OF SAN PEDKO. 185 



Shell of medium size, globose-oblong, equivalve, inequilateral, thin; beaks anterior, small, 

 covered by callous plate; transverse furrow from beak to middle of ventral margin; anterior portion 

 of shell with concentric, wavy, lamellae, which rise to sharp points on the crests of the undulations, 

 the points, taken as a whole, giving the impression of rugose, radiating, raised lines; posterior por- 

 tion of shell gaping, with surface concentrically sculptured; anterior gap large, closed in adult shell 

 by callous plate; with a small mesoplax, the two parts of the protoplax confluent. 



Ditiiensiotis. — Long. 35 mm.; alt. 21 mm.; diam. 20 mm. 



Rather common in the upper Sau Pedro series of San Pedro, Los Cerritos, 

 Crawfish George's, and Deadman Island. 



Living. — Straits of Fuca to Santa Barbara (Cooper): San Pedro (Williamson). 



Pleistocene. — Santa Barbara (Cooj)er) : Harris' ranch, Santa Barbara County 

 (Knecht): San Pedro (Arnold). 



Class SCAPHOPODA.' 

 Order SOLENOCONCHIA. 



Family XL. DENTALIIDiE. 



Genus Dentalium Linne. 



Shell tube-like, gradually tapering posteriorly; longitudinally ribbed; margin of the aperture 

 sharpened; posterior end with an internal, slightly projecting tube, which is provided with a dorso- 

 ventrally elongated opening, the outer layer having a very slight emargination dorsally and ventrally. 



Dentalium elephantinum Linn, is a characteristic species. 



139. Dentalium hexagonum Sowerhy. 



Dentalium hexagonum Sbv., Thes. Conch., Vol. Ill, p. 103, fig. 10. Carpenter, Brit. Assn. 



Rept., 1863, p. 648. Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 238. Keep, 



West Coast Shells, p. 114, 1892. Williamson, Proc. U. S. Nat. Mus., Vol. XV, 1892, 



p. 194. 

 Dcntaliicm neohcxagojium Sharp & Pilsbry, Tryon's Man. Conch., Vol. XVII, p. 19, PI. XI, figs. 



74-86, 1898. 



Shell small, curved, thin, tapering posteriorly; ape.x truncate; surface encircled by fine incre- 

 mental lines and generally by one or two irregular encircling grooves showing interruption in growth; 

 six prominent, equidistant, longitudinal ridges begin at posterior end and become obsolete near 

 aperture; cross-section near posterior end, hexagonal; aperture subcircular. 



Dimensions. — ^Long. 33 mm.; diam. 3 mm. 



Somewhat resembles a miniature elephant's tusk in shape. 



Bather common in the lower San Pedro series at Deadman Island and San 

 Pedro; and in the upper San Pedro series at San Pedro, Deadman Island, Crawfish 

 George's, Los Cerritos, and Long Beach. Found in the Pleistocene at Barlow's 



' The classification adoptod for this divisioa is that used by Dr. W. N. Dall in Part II of the "Contributions to the Tertiary Fauna 

 of Florida." The generic depcription is from Tiyon's "Structural and Systematic Conchology." 



( 24 ) March i, 1903. 



