13 



section, greatest diameter about one twelfth to one fourteenth the length. Sculpture, six equal, 



strong, rounded longitudinal ribs, becoming a little broader and flatter toward 



the aperture and separated by flat or slightly concave interstices. At a distance 



of about 25 mill. from the apex there appears in the middle of each interval a 



fine and inconspicuous, interstitial riblet. Lower down, some times two or three 



or more small riblets are intercalated near the first. Apex entire, hexagonal ; 



anterior aperture hexagonal, the sides flat or nearly so. 



Length 50 mill.; greatest diameter 4 mill. 



Fig. 8. 

 Anterior aperture 

 of D. hexagonum. 



Gould's diagnosis is as follows : Shell long, slender, bony, arcuate, hexagonal with obtuse 

 laterally compressed angles, the interspaces unsculptured ; peristome .si.x-angled. Length 55, 

 diam. 4 mill. 



Distribution; Hongkong, Singapore, Japan. 



This species must not be confused with the West-American DcHtaliiiin neohexagonum 

 Sharp & Pilsbry, which in many collections goes under the name of Dentalium hexag07ium 

 Sowerby. D. jieohexagomim has a length of about 35 mill., with a greatest diam. of 2,5 mill., 

 and sculptured with six strong rounded projecting ribs, losing in prominence on the larger part 

 of the shell, the anterior half being rounded or only faintly hexagonal. 



Var. sexcostatiini Sowerby. PI. VI, fig. 2. 



1860. Dentalium sexcostatiun Sowerby, Thes. Conch. III, p. 103, pi. 223, fig. ii. 

 1872. Dentalium sexcostatum Sowerby, Conch. Icon. pi. 2, fig. 11. 



1875. Dentalium sexcostatum Sowerby. E. A. Smith, Ann. Mag. N.H. (5) XVI, p. 113. 

 1897. Dentalium hexagonum var. sexcostatum Sowerby. PiLSBRY & Sharp, Manual of Conch. 

 vol. XVII, p. 19, pi. 2, figs 27, 28. 



Diagnosis. Shell slender, its smaller half well curved, larger half nearly straight; 

 white, sculpture of 6 very strongly projecting rounded ribs about half as wide as 

 their interstices; the latter on the smaller third of the shell concave and plain, 

 beyond that, one or two interstitial threads appear in each interval on the 

 convex side, and later in those on the concave side ; these increasing in number 

 until near the aperture there are 3 — 6 unequal threads on the flat ground of 

 each interval. Aperture hexagonal, the angles more or less projecting. Anal 

 orifice a minute ovate foramen, excentric in position on the star shaped apex; 

 no slit or notch. Length 62, breadth and length of aperture 5 mill. 



Fig. 9. 

 Anterior aperture 

 of D. hexagonum 

 var. sexcostatum. 



Distribution. Japan, Cape Shima, 18 fms; Goza harbor, 6 fms (St. John); China 

 (Sowerby). 



Remarks. This difters from typical hexagonum in the more sculptured intervals, thicker 

 ribs and larger size, but we have little doubt that the forms intergrade (Pilsbry & Sharp). 



No specimens were obtained by the Siboga Expedition. 



