Genus Rangitotoa n. gen.
Type: Rangitotoa insularis n. sp.
Odhner, in his systematic key of the Ellobiidae (1925, pp. 1–15), which is based largely upon dental characters, has divided the family into six subfamilies—the Carychiinae, Melampodinae, Pedipedinae, Pythiinae, Cassidulinae, and the Ellobiinae.
The first three of these form a major group, having the radula characterised by pectinate marginals with one or more ectocones, while the remaining three subfamilies are grouped together on the common character of blunt chisel-shaped marginals without ectocones.
The new genus Rangitotoa, here described, has a type of dentition characteristic of the first group of subfamilies, but the dentition is nevertheless distinctive, and not in accord with that of any genus belonging to the first group of Odhner's subfamilies.
The dentition suggests a position for these shells between the Carychiinae and the Melampodinae, nearer, no doubt, to the latter. The small dental formula, together with the obsolescent tricuspid nature of the central tooth, indicates a position not much in advance of the Carychiinae, and yet not as specialised as in the Melampodinae, in which all traces of the tricuspid central have vanished.
With regard to the laterals, the large mesocone* recalls the Melampodinae, but the presence of both a definite ectocone and an obsolescent entocone are features discordant for that subfamily.
The shell, however, is not unlike the West Indian Tralia, (Melampodinae), but it lacks the characteristic thickened process within the outer lip, which is a feature of that genus.
[Footnote] * The main or middle cusp of lateral and marginal teeth is termed the mesocone, while cusps situated on the inner side are known as entocones and those on the outside as ectocones.
Rangitotoa insularis n. sp. (Fig. 1).
Shell minute, semi-transparent, imperforate, thin, and colourless. Outline ovate-cylindrical, spire conical, less than half height of aperture. Whorls seven, slowly increasing at first, including a minute smooth protoconch of one flattened whorl. Aperture long and narrow. Outer lip thin and sharp, without a ridge or any processes within. Inner lip with three plaits, uppermost weak and not plainly visible from without, situated at a little above half the height of aperture. Central plait large and prominently projecting; lower plait less prominent, merging below into the thickened narrowly rounded basal lip. Surface smooth except for faint irregular axial growth lines.
Dentition (Figs. 5 and 6): Radula very minute, the detail of the teeth being difficult to see even with a sixteenth oil immersion objective. Dental formula small (12–16) + 1 + (16–12). Central tooth with one large cusp and an obsolete denticle on each side of it; laterals 16, each with a mesocone and an ectocone; marginals 12, each with an entocone, a mesocone, and from two to four ectocones.
Height, 3.7 mm.; diameter, 1.9 mm. (holotype)
" 2.9 mm.; " 1.4 mm. (average size specimen).
Holotype: In Auckland Museum (collected by A. W. B. P., March 12, 1930.
Habitat: Rangitoto Island; type from near the “Beacon.” Found living only near high-water mark, on the under sides of stones which are partly in contact with mud. The writer has found this species to be distributed around the southern and western sides of Rangitoto Island, but so far has failed to discover it elsewhere.