92 DENTALIUM-GRAPTACME. 



grown shells are striate throughout ; and in some of the old ones 

 less than a third of the shell is sculptured. In form there is also 

 considerable variation, occasional examples fully j ustifying Philippi's 

 term " subrecta," while others are very markedly arcuate. 



We have noted above the essential identity of this form with the 

 Antillean D. semistriatum Turton. In the average, a greater por- 

 tion of the tube is striated in this than in D. semistriatum; but in 

 many individual specimens this does not hold, and they are quite 

 indistinguishable. The West Coast form is, at most, only a sub- 

 species. 



The original description of this species is as follows : 



" Shell whitish, polished ; posterior end somewhat recurved, very 

 finely striated ; no posterior slit. Length 1-4, diam. O'l inch. The 

 very fine striae continue about half the length of the shell, which is 

 rather narrower in proportion than D. nebulosum " (B. & &). 



The habitat of the type was unknown, but as a large number of 

 the shells described in Broderip and Sowerby's paper, cited above, 

 were from the west coast of Mexico, it is very probable that the 

 original semipolitum came from thence, especially as their descrip- 

 tion agrees perfectly with specimens from that region. 



It has also been well-described by Philippi as D. hyalinum, and 

 young shells, in which the striae extend from end to end, have re- 

 ceived the names lirulatum Morch and liratum Carpenter. All 

 published information upon the latter two forms is given below. 



D. lirulatum Morch. Shell arcuate, dilated toward the aperture, 

 apex attenuated ; thin, white, very closely lirulate with beautiful 

 regularity, the interstices deep, milk-white ; growth-striae wanting. 

 Length 8, diam. 1'25 mill. (MoercTi). 



Gulf of Nicoya, west coast of Costa Rica (Oersted). 



Has the form of D. acuminatum Deshayes. It is probably a 

 young shell of the D. semipolitum group, and nothing in the diag- 

 nosis precludes the supposition that it is identical with that species. 



D. liratum Carpenter. Shell solid, white, cylindrical, little 

 curved, very closely covered with very delicate longitudinal line 

 numbering in the young about 12, in the adult about 30 ; they are 

 acute and hardly of equal size. Branchial [anal] orifice simple. 

 One perfect though rather small specimen was found entangled in 

 the byssus of Modiola capax; fragments occurred of a much larger 

 size. Length -25, breadth -011--03 inch [L. ^'25, greatest diam. 

 O75, apical diam. O275 mill.]. 



