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3. New Classification.
The generic grouping here proposed for New Zealand Volutes is put forward with considerable hesitation, for other schemes could be drawn up with perhaps as weighty arguments. A statement of the principles governing this classification will, at all events, make the writer's position clearer.
(1.) The shape of the nucleus is a valuable guide in grouping related shells. Allowance must be made for exceptionally early or late calcification, but where a number of individuals can be observed a general form is apparent.
(2.) The number of columellar plaits is often variable, but there are always fairly strict limits. Thus, among the dozens of specimens of A. gracilis examined, only one had other than 4 plaits; in the new genus Mauia no variation in number was seen except in the species M. insignis, and hardly any in several other species: e.g., A. finlayi. Members of Spinomelon have nearly always 5 or 6 folds, 4 being quite exceptional.
(3.) The anterior notch has considerable systematic value. A shallow notch is a primitive character, and deepening marks evolutionary progress.
(4.) The primitive sculpture of most of the New Zealand Volutes consists of strong, smooth, sharp axial ribs. Remains of an early spiral ornamentation are occasionally seen, but do not assume any importance. Evolution from simple axial ornamentation may follow two paths:—
| (a.) |
The costae may become angled and then tubercular: e.g., the line Waihaoia allani or W. thomsoni—bathgatei—phymatias—dyscrita. |
| (b.) |
The costae may become obsolete and the shell have a smooth surface: e.g., the line Waihaoia allani—pretiosa—cognata. |
Often a tendency is manifested to revert to axial costae or tubercles after a smooth stage has been reached: e.g., Spinomelon benitens—parki—speighti. This is also shown in the ontogeny of large examples of Alcithoe swainsoni, where the penultimate whorl is smooth but the body-whorl shows a return to tubercles.
(5.) The independent orthogenetic evolution of these characters in different groups produces combinations showing a considerable degree of parallelism.
The use of five genera, in addition to Neoplejona and Lyria, is proposed: Alcithoe, Mauia, Waihaoia, Metamelon, and Spinomelon, the last four being new, and Waihaoia having the two new subgenera Teremelon and Pachymelon.
Mauia and Waihaoia are represented in the greensand of Kakahu and Waihao, probably Eocene; Metamelon and Spinomelon are not known until the Wharekuri greensands, perhaps Middle Oligocene; but Alcithoe does not appear until the Awamoan—i.e., Upper Oligocene or Lower Miocene. Mauia is proposed for an early line of shells with a deep anterior notch and a ridged fasciole. Gerontism soon developed, and the suture ascended so high that most of the spire was ultimately embraced by the body-whorl. Waihaoia is founded on the small W. allani, which has simple axial ornamentation, a scaphelloid nucleus, 5 columellar plaits, and a quite shallow anterior sinus. With it are included other shells from a lower or equally low horizon, with more advanced sculpture and 4 plaits, but still with the shallow notch. A series from Clifden described by Finlay seems most conveniently to be classed here. W. bathgatei has a higher spire than W. allani, the axials are slightly angled, and the anterior notch is deeper; the columella, however, is the same. In W. phymatias the spire-whorls are as in W. bathgatei, but on the body-whorl the axials develop into prominent
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tubercles, and the shape is rather broader; the columellar folds are only 4 in number, but the notch is no deeper than in W. bathgatei. W. dyscrita is much broadened, and the ornamentation is accelerated so that most of the whorls are tubercular; the columella has 4 plaits, and the anterior notch has not advanced any farther. This development was only an offshoot from the main stock, which, except for the shortened beak, is still much the same as the Eocene W. thomsoni in the Miocene W. obsoleta.
The new subgenera Pachymelon and Teremelon are proposed for specialized groups; the former have a thick test and a fairly shallow notch; the latter are thin and graceful, for the most part smooth and shining, notch moderate, fasciole not strongly marked. These subgenera, especially Pachymelon, show considerable likeness to Spinomelon, but they have not the spiked nucleus, therefore they have been retained under Waihaoia.
Spinomelon (type Lapparia parki) has been introduced for a number of large shells of oval shape with axial corrugations, a spiked nucleus, 5 or 6 columellar folds, and a moderate to fairly deep anterior notch. The sculpture and the columellar folds are the same as in Waihaoia allani—indeed, the general appearance of the two species is so similar that one is almost forced to the conclusion that they are directly related.
The species grouped under Metamelon have also a spiked nucleus, but they are small shells inclined to gerontism, and are known as early as Spinomelon. The beak is always produced, the anterior notch very deep, and the base of the body-whorl contracted.
The origin of Alcithoe, the only living genus, is not clear. The small Mauia angusta has many of the characters of Alcithoe, but has a rather better developed anterior notch, and the protoconch is not known. Spinomelon is a close relative, but whether each is an independent offshoot of Waihaoia, whether Spinomelon gave rise to Alcitho or whether the origins are from other stocks than at present known, are difficult questions. Each has something in its favour.