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The Atoll of Funafuti, Ellice group : its zoology, botany, ethnology and general structure based on collections made by Charles Hedley of the Australian Museum, Sydney, N.S.W.
Rissoina exasperata, Souverbie. — (Fig. 10)
Rissoina exasperata, Souverbie.
(Fig. 10).
Souverbie, Journ. de Conch., xiv., 1866, p. 259, pl. ix., fig. 10.
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Fig. 10.
To this species is referred with doubt a series from Funafuti. The published account is insufficient for accurate determination, and my principal reason for considering the Ellice shell to be R. exasperata is its identity with a common New Caledonian shell which I have myself collected at Panie, N.C., and have received from Noumea, from Mr. R. C. Rossiter, That Conchologist regards it as JR. exasperata, and it answers fairly to Souverbie's description as far as that goes, but it is less easy to reconcile it with his figure.
* Conchylien Cabinet, i., 22, 1885, p.54 pl. xiv., fig. 10.
As a synonym I would add the name of Rissoina quasillus, Melvill and Standen† from Lifu. Neither figure nor description of this are sufficient for decision, we are not told how many ribs there are, whether continuous or discontinuous, etc., yet there seems nothing incompatible between R. quasillusand the shell under discussion. That these authors should have failed to institute a comparison between their supposed novelty and a shell so similar from the same locality, suggests that they overlooked Souverbie's description.
Since so much confusion has enveloped R. exasperata, it is not superfluous to present a drawing (Fig. 10) and remarks upon the Funafuti specimens.
Shell elongated, when well preserved slightly turriculated, varies slightly in being more slender or more stout. Dead shells are white. A fresh specimen has within the aperture four narrow, spiral lines of golden brown; outside, another such line colours the anterior spiral lyra of the antipenultimate whorl, two such the second and third of the penultimate, and three such the second, third, and fifth lyræ respectively of the ultimate whorl. Other worn specimens show traces of this colour pattern. On the last whorl there are nineteen or twenty stout, narrow, erect, longitudinal ribs, half the breadth of their interstices; these arise at the suture, and maintain an even size to the base, on attaining which they suddenly cease. These ribs are repeated on the preceding whorls; they are not continuous from whorl to whorl, but each arises and ends between the projections of predecessors and successors. They are fewer and relatively stronger on the earlier whorls, being indicated on the second and fully developed on the fourth.
On the last whorl there are five spiral cords, which are half the height of the longitudinal ribs. At the point of intersection a bead arises on the ribs. The hollows in the lattice work thus formed are square and are minutely spirally striated. The base is encircled by two or three small and finely beaded lyræ. Three spiral cords ascend for three whorls, growing weaker as they proceed. The first whorl is dome-shaped, and the second keeled.
These specimens are 2½ to 3½ mm. long, and have seven to eight whorls.
Occurred in the lagoon in shallow water.
* Tryon —Man. Conch., ix 1887, p. 384, pl. lvii., fig. 96.
† Melvill standen — Journ. Conch., viii., 1897 p.308, pl. xi. fig. 65.
Though certainly distinct, R. transenna, Watson, has much resemblance to this species. R. clathrata, Adams, appears to differ slightly by coarser sculpture.