Affinities.—This species differs considerably from the first two of the genus. In several characters it approaches nearer than the other species to [Tetraclita], especially to [T. porosa];—namely, in the scutum having an adductor ridge and crests for the lateral depressores, in the whole form of the tergum, in the thick walls liable to much corrosion, in the narrow radii, and in their edges, as well as those of the alæ, being crenated; and, lastly, in the character of the cirri, more especially of the third pair, with its coarsely pectinated spines. It also approaches, in all its characters, [Balanus imperator] and [flosculus].
4. [ELMINIUS] SIMPLEX. Pl. [12], fig. [3].
Shell ribbed longitudinally, dirty white; radii extremely narrow, smooth-edged; scutum having an adductor ridge.
Hab.—New South Wales (Sydney and Twofold Bay); Van Diemen’s Land; tidal rocks, often attached to other Cirripedes, and associated with [Balanus nigrescens], [Tetraclita purpurascens], [Catophragmus polymerus]; Mus. Brit., Cuming, Stutchbury, and Darwin.
This species, of which I have seen specimens from the above three localities, all exactly agreeing with each other, is perhaps the Australian representative of [E. plicatus], which seems to be confined to New Zealand.[113] In all essential points it comes so near that species, that I shall make the greater part of my description comparative.
[113] I am bound to state that I have seen two specimens of [E. plicatus] marked Sydney, and one marked Moreton Bay, but in both cases the collectors had visited New Zealand, so that a wrong habitat by mistake might easily have got attached to the specimens in question.
General Appearance.—In external appearance there is considerable difference from [E. plicatus], for [E. simplex] is generally of a regular conical shape, of a dirty-white colour, with the surface well preserved, having moderately wide, not very prominent longitudinal ribs. The orifice is rather small and pentagonal. The radii are extremely narrow or linear, with quite smooth edges; the sutures, however, are always very distinct, and in the upper part, the alæ are generally rather widely exposed, as viewed from the outside. The largest specimen which I have seen was .7 of an inch in basal diameter.
The opercular valves are closely similar to those of [E. plicatus], but the scutum is generally a little more elongated, and the articular furrow not so deep: in accordance with this last fact, the articular ridge in the tergum is not so prominent as in [E. plicatus]; but we have seen that these several characters are highly variable in [E. plicatus]. The slope of the basal margin of the tergum towards the spur varies in the present species, in a strictly analogous manner, as it does in [Tetraclita porosa].