DENTALIUM-GRAPTACME. 85 



D. clathratum E. VON MARTENS, Sitzungs-berichte der Gesell- 

 schaft naturforschender Freunde zu Berlin, Jabrgang 1881, p. 66, 

 (April, 1881). 



Similar to D. compressum Watson of the West Indies. The in- 

 terspaces between the ribs are about three times as wide as the ribs 

 themselves, where smaller riblets are not interposed. (Martens'). 



Subgenus GRAPTACME Pilsbry & Sharp, 1897. 



Surface sculptured with close, fine, deeply engraved longitudinal 

 striae near the apex, the remainder smooth ; or rarely the striae per- 

 sist half or all the length. Moderate sized or small species, cylin- 

 drical in section, and white or nearly so. 



Distribution : Antillean and Panamic regions ; Indo-Pacific ; 

 mainly living in quite moderate depths. 



There is remarkable diversity in the characters of the apices in 

 this apparently very natural group. In some species, as D. semi- 

 striatum and eboreum, the apical orifice is perfectly simple ; in 

 others, such as D. aciculum, a slight notch appears ; in D. leptum 

 there is a slit on the convex side. D. sericatwn has the slit lateral, 

 and it is on the concave side in D. inversum. D. sectum and calamus 

 have the apical aperture reduced to a slit across the summit of a 

 convex apical plug, an altogether peculiar structure. 



In old or worn specimens of some species the striation is lost. 



I. Apex with simple orifice or variously slit, not contracted, 



Group of D. semistriatum. 



II. Apical orifice contracted by a callous plug cleft by a slit from 

 convex to concave side, Group of D. sectum, p. 96. 



GROUP OF D. SEMISTRIATUM. 



I. No slit at the apex ; apical seventh to two-thirds with fine, 

 clear cut incised strise, the remainder smooth and polished. 



a. Length 76 mill., about 18 times the diam. ; posterior 



third striate, novcehollandice, p. 93. 



a'. Length 20-31 mill., 10-12 times the diam., eboreum, 



p. 89 ; semistriatum, p. 90 ; semipolitum, p. 91 ; aciculum, 



p. 93. 



II. An apical slit or notch on the side ; young zig-zag clouded with 

 opaque white ; length 33-35 mill., 9-10 times the diam., 



sericatum, p. 86. 



