1890.] MARINE MOLLUSCA OF ST. HELENA. 257 



men has eight whorls, of which the three apical are white, convex, 

 and ornamented with subdistant, very oblique, arcuate, and very- 

 slender lirse. The remainder of the shell is of a whitish-wax tint. 

 The length is 7| millim., the diameter 3. 



The more extended series of specimens at hand now shows that 

 PI. helenensis, which I described at the same time as PL gemma, is 

 only a variety of this species, differing chiefly from the typical form in 

 colour. 



Some examples are entirely white, others of a uniform rich brown, 

 whilst others are of intermediate tints. Some have all the upper 

 ends of the costse of a reddish colour, some only the alternate ones ; 

 and the transverse zone on the body-whorl is not constantly pre- 

 sent in all specimens ; indeed it appears to be mostly wanting in the 

 yellowish and brownish examples. PL lavalleana, d'Orbigny, is the 

 West-Indian representative of this species, from which it differs in 

 the position of the colour-band, its more attenuated body-whorl, and 

 in the different position of the angle of the volutions. 



Pleurotoma (Mangilia) mellissi. (Plate XXI. fig. 5.) 

 Testa parva, fusiformi-ovata, dilute fuscescem ad suturas fusco 

 tincta, circa medium anfr. uliimi fusco zonata ; anfractus 6|, 

 primi 2| convexi, Iceves, tertius convexus, ohliquiter tenuissime 

 liratus, cceteri superne declives et angidati, costis tenuihus circa 

 14 et liris graciliorihus spiralibus (in anfr. superionhus 3, in 

 ultimo 16-20) cancellati, imdique minute squamoso-striati ; 

 apertiira angusta, longit. totius h vix cequans ; lahrum incras- 

 satum, intus Iceve ; sinus haud pjrofundus. 

 Longit. 5 millim., diam. 2. 



The microscopic structure of this species is very like that occurring 

 in PL subqxiadrata, and appears under the microscope to consist of 

 numerous spiral series of very minute grain-like scales, which, at 

 times, are arranged one under the other, so as to produce the 

 appearance of longitudinal series also. It may be known from 

 PL subquadrata by its finer cancellation and the more central posi- 

 tion of the colour-zone upon the body-whorl, which also is not con- 

 tracted in the same way below the middle. 



Pleurotoma (Clathurella ?) commutabilis. (Plate XXIII. 

 fig. 3.) 



Testa p)arva, fusiformi-ovata, aut alba , flavescens, aut lilacea, costis 

 longitudinaJibus ad 12 lirisque transversis (mi anfr. superiorihus 

 2-3, in ultimo 8), fortissime cancellata ; anfractus 5^, pi imi 1| 

 vitrei, pecidiares, superne carinati et concavi, quasi truncati, re- 

 liqui 4 convexiuseuli ; apertura parva, angusta, longit. totius | 

 adcequans ; labnim vix incrussatum ; canalis brevissimus,latus ; 

 sinus parvus, inconspicuus ; columella rectiuscula, tubercuhs 

 duobus prop>e medium munita. 



Longit. 4 millim., diam. 1|. 



This species is very distinctly characterized by its form and the 

 coarse style of its sculpture. The ribs and lirae are about equally 



