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1. Genus Monalaria n. g.
Genotype: Struthiolaria tuberculata subsp. concinna Suter, 1917.
Shell somewhat small, ovate, umbilicus closed in the adult, conchwhorls at first spirally lirate, later with curved axial ribs, and finally keeled and tuberculate; outer lip reflexed, thickened, concave above, then produced in a sweeping curve into a broad rounded wing opposite the lower keel, columella straight, aperture produced into a short widelyopen canal.
This genus differs from Struthiolaria in the contour of the outer lip, and the presence of a straight columella.
Monalaria concinna (Suter), 1917. (Plate 11, figs. 1, 2, 3.)
1917. Struthiolaria tuberculata Hutton subsp. concinna Suter, N. Z. Geol. Surv. Pal. Bull. No. 5, p. 9, pl. ii, fig. 9.
Shell rather small, ovate; spire broad, gradate, a little over half the height of aperture; whorls 6, later ones strongly shouldered; sculpture, first 2 conch-whorls convex, with 5 strong but narrow spiral ridges with wide interspaces, on third whorl they are reticulated by curved axials slightly stronger and wider apart than spirals, 3 posterior spirals much finer than other 5; fourth whorl strongly angled with wide shoulder, a fairly strong spiral thread on angle, 3 above and 2 below of equal sterngth, and, between these, 2 finer spirals with wide interstices in all cases; there are 13 strong rounded axials which commence a short distance from suture and are arched, anterior end being slightly in advance, they are not so
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strongly curved as axials of earlier whorl, but are much more prominent; on the penultimate whorl spirals are same as before, but axials have rather the appearance of tubercles on angle of shoulder; body-whorl is spiralled by fine regular threads with wider interstices, the row of tubercles seen on the penultimate whorl continues with unabated strength, and below this is a double keel consisting of 2 rows of low tubercles which do not correspond to those of shoulder nor with each other; suture impressed; aperture inclined, ovate with a short truncated canal below; outer lip reflexed, thickened, concave above, but well produced at lower keel, retreating somewhat rapidly in a shallow sinus to anterior canal; inner lip very moderately calloused; columella straight, ending in a short beak.
Holotype in the collection of New Zealand Geological Survey.
Height, 31 mm.; diameter, 23 mm.
Localities.—Waihao greensands (holotype, J. A. Thomson); 176, 933, Black Point, Waitaki Valley; 164, greensands above coal-beds, Kakahu; 487, above coal-beds, Ngapara; 27, roof of upper coal-seam, Ten-mile Creek, north of Grey River.
The last two identifications are based on casts, and so may be of shells slightly different from concinna, but as far as can be seen they are specifically identical. It will be observed that the beds at all these localities are of a uniformly low horizon, so that this species will be of great value for zoning purposes because of its wide distribution.
For the subspecific relationship with Struthiolaria tuberculata nothing can be put forward as evidence. The shells are far apart; indeed, the apertures are so different that the distinction is of generic importance.
Several specimens show an earlier lip, after the formation of which the animal continued building its shell in the usual way. One such lip on the holotype is a complete whorl behind the present aperture, while a specimen from Black Point has a quite complete thickened lip one-third of a turn behind the final one.
Suter (1917, p. 9) mentions the cast of another specimen showing a fourth row of nodules, and concludes therefrom that S. tuberculata may have two, three, or four keels. This quite ignores other and much more important characters, for the cast with the four nodules is that of a Galeodea cf. senex (Hutton).
Monalaria minor (Marshall). (Plate 11, figs. 5, 6, 7.)
1917. Struthiolaria minor Marshall, Trans. N. Z. Inst., vol. 49, p. 451, pl. 34, figs. 12, 13.
Localities.—Wangaloa (type); Boulder Hill, near Dunedin (H. E. Fyfe).
The exact horizon with reference to the European time-scale has not yet been worked out, but it is probably lowest Tertiary. (For description of the sculpture, see above, p. 164.)