2. [ELMINIUS] MODESTUS. Pl. [12], fig. [1 a]-[1 e].
Shell folded longitudinally, greenish or white: radii of moderate breadth, smooth edged: scutum without an adductor ridge: tergum narrow, with the spur confluent with the basi-scutal angle.
Hab.—New South Wales; Van Diemen’s Land; New Zealand; very commonly attached to littoral shells and rocks; associated with [Balanus trigonus] and [vestitus]; Mus. Brit., Cuming, Stutchbury, Darwin.
General Appearance.—Shell conical, generally rather steep, occasionally depressed: walls longitudinally folded, sometimes very deeply, sometimes only to a slight degree: colour dull greenish or white. Radii of moderate width, with their summits very oblique, smooth and slightly arched: alæ much exposed, with their summits less oblique than those of the radii: the portion added to the alæ during the diametric growth differs much in appearance from the other portion. The scuta have the growth ridges but little prominent; they are crossed by a faint longitudinal band of gray. The largest specimen out of the many which I have seen, was under .4 of an inch in basal diameter.
Scuta, destitute of an adductor ridge and of crests for the depressor muscles: the articular ridge is moderate; but the articular furrow is rather wide: the internal occludent margin is much thickened. The Terga are narrow and small; they are somewhat variable in shape, caused by the degree to which the basal margin is hollowed out (fig. [1 c]-[1 e]), and likewise by the extent to which the upper end of the valve has been worn away. No spur is apparent, for it is confluent with the basi-scutal angle of the valve. The articular ridge is very prominent, and runs down to the basi-scutal angle; and as the valve in this part is extremely narrow, with the spur not developed, it here assumes a channelled structure. The basi-carinal corner of the valve is furnished with rather feeble crests for the depressor muscles, and in those varieties in which the basal margin is much hollowed out, this part is remarkably narrow.
Structure of the Parietes and Radii.—The internal basal edges of the parietes and the sutural edges of the radii and alæ, are all smooth. The lower edge of the sheath depends freely. In the green varieties the colour is most distinct on the internal surface of the shell. The four compartments separate very easily when the shell has been ill preserved in spirits, or after a very short immersion in caustic potash.
Mouth, as in [E. Kingii], excepting that there are only three teeth on each side of the notch (which is deeper) on the labrum. The cirri resemble those of [E. Kingii]; the segments in the sixth pair are equally elongated, and bear five or six pairs of spines.
Affinities.—This species is closely allied to its South American representative [E. Kingii]; the differences consist in its smaller size, often greenish colour, more folded walls, and narrower radii: the internal basal edges, also, of the parietes are here smooth, instead of being striated, as in [E. Kingii]. The terga present even more obvious differences, in their narrowness, channelled under surface, and in the absence of the spur, or more properly in its confluence with the basi-scutal angle of the valve.