ARNOLD — THE PALEONTOLOGY AND STKATIGBAPHY OF SAN PEDRO. 185 



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

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

 of shell with concentric, wavy, lanielke, which rise to sharp [joints on the crests of the undulations, 

 the |)oints, 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 meso|)la.\, the two parts of the protoplax confluent. 



Di/nenstons. — Long. 35 mm.; all. 21 mm.; diam. 20 mm. 



RiitluT common in the upper S;iii Pedro .series of San Pedro, Los Cerritos, 

 Cnuvlisli George's, and Dead man Island. 



Living. — Straits of Fuca to Santa Barbara (Coopcu): San J'edro (Williamson). 



Pleistocene.— Siantn Barbara (Cooper): Harris' ranch, Santa Barbara County 

 (Knecht): San Pedro (Arnold). 



Class SCAPHOPODA.' 

 Order SOLENOCONCHIA. 



Family XL. DENTALIID^. 



Genus Dentalium Linne. 



Shell tube-like, gradually tajjering posteriorly; longitudinally ribijed; 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 elcphantbium Tiiiiii. is a characteristic species. 



139. Dentalium hexagonum Sou-erhy. 



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



Rept., 1863, p. 648. Cooper, 7th Ann. Rept. Cal. St. Min., 188S, p. 238. Keep, 



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



p. 194. 

 Dentalium neohexagonnm Sharp & Pilsbry, Tryon's Man. Conch., Vol. XV'II, p. 19, PI. XI, figs. 



74-86, 1898. 



vShell 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. 



llather 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 Lon<;' Beach. Found in liie Pleistocene at Barlow's 



' Tlie clussincatioii ndoptod for this division is thnt used by Dr. W. N. Dall in P-irt II of the "Contributions to llie Ttrliai y Fauna 

 of Florida." The generic deBcription is from Tiyon's "Structural and Systpmatic Conchology." 



( 24 ) March 4, IQCj. 



