OCR Text |
Show 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 lirae. 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 PI. 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 costae of a reddish colour, some only the alternate ones; and the transverse zone on the body-whorl is not constantly present in all specimens; indeed it appears to be mostly wanting in the yellowish and brownish examples. PI. 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 fuscescens ad suturas fusco tincta, circa medium anfr. ultimi fusco zonatct; anfractus 6|, primi 2^ convexi, Iceves, tertius convexus, obliquiter tenuissime liratus, cceteri superne declives et angulati, costis tenuibus 14 et liris gracilioribus spiralibus (in anfr. superioribus 3, ultimo 16-20) cancellati, uncliqite minute squamoso-striati; apertura angusta, longit. totius | vix cequans; labrum incrassatum, intus lave ; sinus haud profundus. Longit. 5 millim., diam. 2. The microscopic structure of this species is very like that occurring in PL subquadrata, 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 position of the colour-zone upon the body-whorl, which also is not contracted in the same way below the middle. PLEUROTOMA (CLATHURELLA?) COMMUTABILIS. (Plate XXIII. fig. 3.) Testa parva, fusiformi-ovata, aut alba, fiavescens, aut lilacea, costis longitudinalibus ad 12 lirisque transversis (in anfr. superioribus 2-3, in ultimo 8), fortissime cancellata ; anfractus 5|, pi imi 1^ vitrei, peculiares, superne carinati et concavi, quasi truncaii, 4 convexiusculi ; apertura parva, angusta, longit. totius adcequans ; labrum vix incrassatum ; canalis brevissimus, latus sinus parvus, inconspicuus; columella rectiuscula, tuberculis duobus prope medium munita. Longit. 4 millim., diam. Ii. This species is very distinctly characterized by its form and the coarse style of its sculpture. The ribs and lirae are about equally |