to detect distinguishing specific features. Except that in postulatus the spiral ribs are equal to or broader than the interstices whilc in dennanti they are stated to be narrower, there would seem to be little difference between these two very closely related forms.

Now, any genera that contain like forms on opposite sides of the Tasman Sea, whether in times past or present, are full of interest to the stratigrapher and the regional geologist, but not all these genera are of equal value on which to base correlationary hypotheses. For instance, many forms of Dentalium and Natica from Tertiary beds in New Zealand can be almost exactly matched by Australian fossils, but this proves little, for these two evolutionary lines are very long and wide-ranging, and have comparatively narrow limits of specific variability. Although, for example, Natica (Cochlis) notocenica Finlay is with difficulty separable from N. (C) subnoae Tate as a species, yet their ancestors and descendants in each country are more easily separable, and it is unwise to lay much stress on a similarity perhaps brought about by chance.

But when the genera concerned are short-ranging and highly distinctive, the matter is on a different footing altogether. Much less allowance can be made for convergence, environment, etc., and one seems justified in deducing a somewhat near relationship between the species concerned. In the present case, Heligmope satisfies both conditions, and is as excellent an example as could be wished for. Its ancestry is quite problematical, it suddenly appears in both countries at about the same time (Lower Miocene) it gave rise to but few species, all very similar, it is so outstanding a genus that cven its relationships have puzzled every observer, and it very rapidly dies out again. It is, in fact, quite comparable with the American genus Orthaulax, which for much the same reasons has for long been regarded as an “index fossil.” Woodring (Pub. No. 385, Carnegie Institution of Washington, p. 91, 1928) says of it that “the dangers of relying too much on the range of one genus are fully recognised,” but that “Orthaulax has turned out to be particularly valuable” for much the same reasons as I have mentioned in the case of Heligmope. There is, therefore, no reasonable room for doubt that the genus Heligmope must have arrived in one of its countries very soon after its inception in the other. Whether it had its origin in Australia or New Zealand is at present problematical, as it is not yet possible to determine which of the horizons is the older. Chapman and Singleton (Proc. Pan-Pac. Sci. Cong. Australia, 1923, vol. 1, pp. 3 and 8, 1925) place both the Australian localities in the Kalimnan, which they regard as Lower Pliocene; Marwick (Trans. N.Z. Inst., vol. 57, pp. 573 and 576, 1927) puts the Kaawa beds in the Waitotaran, which he places as the lowest of his three Pliocene divisions.

If it is granted, then, that the forms of Heligmope in New Zealand and Australia are practically contemporaneous, and so closely allied that there has been little time for evolutionary change, also that the chance of convergence can be here ruled out, we are left with an interesting deduction. Tate stated that the genus “may be described as an imperforate Eunaticina, or a Sigaretus, with a sinuated

front margin, and stands in relation to it as Protoma does to Turritella.” Now if either this contention, or that of Bartrum and Powell, that the genus is related to Turbo or Tonna, be correct, it follows that there must have been an easy and direct method of connection between the two countries in the Pliocene! This is so radically counter to the opinions of geologists past and present that it is worth careful consideration; yet from the evidence the deductions so far made seem impossible to avoid. Both Naticoids and Turbinid shells are littoral and sub-littoral dwellers and slow progressors, and would be effectually barred by any depth of water; their young are also mostly seaweed and sandy beach frequenters and there is no evidence that they are much carried about by currents—rather the reverse. To allow of their progress over the 1200 miles between New Zealand and Australia, one would have to postulate a connection not only of shallow waters and sandbanks, but also of so direct a path that climatic conditions had little play. The species of these groups are inclined to be so local that the change of temperature produced by but a few degrees of latitude, almost always modifies or exterminates them. The suggestion of alliance with Tonna (inherently improbable however, from shell features) would dispose of a few of the difficulties but not many. Powell (Trans. N.Z. Inst., vol. 57, p. 559, 1927) has recorded the appearance of a Recent Australian species of Tonna (tetracotula Hedley) in New Zealand waters and remarks, “That Recent conditions are still favourable to the immigration of Australian species of certain types (italics my own) is indicated by the three species …. recorded in this paper.” Strangely enough, one of these is a Natica, but it is of a widely spread subtropical type, and differences can be seen between New Zealand and Australian examples, so that it is more likely an immigrant from the north. But even supposing that Heligmope is of Tonnid affinities and followed the same methods of distribution, it is necessary to invoke a favourable ocean current, and this at once brings us to a standstill again. For in the first place any such ocean current influence (as I have already described in the Verbeek Gedenkboek, p. 166, 1925) does not seem to have antedated the Castlecliffian stage. Those species which may reasonably be assumed, from evidence elsewhere and from embryonic characters, to be readily transported by currents^* belong to such families as the Cymatiidae, Muricidae, Architectonicidae, Thaididae, etc., and while the bulk of those that are identical or nearly identical with Australian species occur only as Recent or Pleistocene shells in New Zealand, there are already a few such as Cymatum spengleri, Mayena australasia, and “Typhis” zealandicus Hutton which are present in the Castlecliff beds. But no member of this class of shell occurs, as far as we yet know, at Nukumaru, the horizon following directly below, and even at Castlecliff the Australian influence is negligible compared with

the number of migrants in Recent (and probably Pleistocene) times. Fusitriton, Fusus, Galfridus, Agnewia, Austrotriton parkinsonia and other Cymatiidae, Hydatina, Philippia, etc., are sufficient evidence of a post-Pliocene influx that was not possible during perhaps the whole of the Tertiary period. A suggestive statement in this connection is that made by Marshall and Murdoch (Trans. N.Z. Inst., vol. 52, pp. 117, 118, 1920); “Three miles to the north of Kai-iwi an old landsurface can be seen distinctly in the stratification” (of the Wanganui Pliocene series) …. “There is another instance …. near the south end of the Nukumaru Beach …. they show that there were temporary oscillations in the level of the land whilst the deposition of these rocks was in progress.” Another suggestive fact is the statement made by Hodge Smith and Iredale (Proc. Roy. Soc. N.S.W., vol. 58, pp. 157, 168, 1924) that “the nature of the evidence of a negative movement of the strand line of four hundred feet in New South Wales is both geological and biological …. A southern shore-line coexistent with the East Coast of Tasmania was drowned when Bass Straits were formed …. This …. took place certainly not earlier than Pleistocene and probably in quite recent times.” I suggest that here are possible reasons for the formation of the present ocean current system in the Tasman Sea, and an explanation of the preponderatingly Recent occurrence of Australian migrants in New Zealand waters.

To return to Heligmope: in the second place, if such a favourable current is invoked in Waitotaran times, the question at once arises, Why did not other forms come across too? The gritty or sandy shallow water nature of the Kaawa Creek (and also Nukumaru) beds is much the same as the calcareous sands at Muddy Creek, (Upper) and Hallett's Cove, so that, apart from the (possibly) slightly colder climate, the ecological controls should have been fairly similar. But, so far as I know, not a single other species from Kaawa Creek is closely enough related to an Australian Kalimnan species to be worth comment; the species of such easily distributed genera as Terebra, Murex, Olivella, etc., belong to quite different lines of descent, and no Cymatiidae at all are present.

The ocean current hypothesis, then, is in this case out of the question, and once more we are brought back to the deduction that unless the animal of Heligmope was radically different in its habits from that of other shells of similar shape and approximate appearance, it seems inevitable that a direct connection across the Tasman Sea existed at some time in the Lower Pliocene. Yet this invites one of the same objections as the ocean current idea; why did not many other sub-littoral forms accompany Heligmope across and become established in New Zealand? Is any solution evident to this perplexing problem?

I have tried to build up the evidence in favour of this Pliocene connection not because I believe in it, but because I wish to demonstrate the danger of arguing thus about any genus, useful and outstanding though it may appear, before its natural affinities are truly known, or while any other interpretation can be given. There is one point about Heligmope that no observer has yet mentioned, and in

the absence of this one small but all important detail, the problem as I have outlined it would have to be left unsolved in an unsatisfactory and perplexing state. But I put forward a very simple solution. I suggest that Heligmope belongs to the Ianthinidae, and that the whole ensemble of shell features conclusively proves this. Ianthina has a very characteristic sinus on the periphery (though its near ally Recluzia has none), and at first sight Heligmope, in lacking this, differs considerably in the aperture and lines of growth. But push the sinus of such a species as Ianthina ianthina down on to the base, half-way between the columella and the periphery, and prolong the aperture downwards instead of sideways to correspond, and a very close approximation to Heligmope is obtained. Every other feature of Ianthina is present—the twisted pillar with the little basal lip and the thin spread of parietal callus; the same suture and prominent growth lines, more lamellose posteriorly; the obscure wide spiral ribs, though much reduced in Ianthina, especially above the periphery; and the thin fragile shell, still showing traces in some specimens of postulatus of the characteristic dark bluish-violet colour! Bartrum and Powell were unable to record the protoconch; I suggest that when a perfect specimen is found, the apex will turn out to be as in Ianthina, glassy, conically dome-shaped, of two or three whorls set quite oblique to the axis of the shell, and more or less immersed in the succeeding shell whorl.

The placing of Heligmope as a Tertiary genus of the Ianthinidae of course removes the difficulties arising out of its distribution. Ianthinas of the two groups danthina-violacea and exigua-capreolata occur fairly commonly all round Northern New Zealand and are washed up on every Australian coast. A wide distribution is to be expected in all pelagic animals, and if Heligmope lived in the open waters of the Tasman Sea in the middle and late Pliocene, it is not curious, but normal, to find it in the shallow water beds of both countries. No connection of any sort need be postulated, and there is no longer an annoying problem.

Another matter of interst is that Heligmope postulatus (Bartrum) seems to have an ancestor in New Zealand in Lippistes pehuensis Marwick (Trans. N.Z. Inst., vol. 56, p. 319; Pl. 73, Figs. 6, 8, 1926). This has the same sculpture and basal sinus, but the spire is so much lower as to be sunk below the body whorl, so that the shape of Lippistes is simulated. I have previously (Trans. N.Z. Inst., vol. 57, p. 396, 1926) placed this species with the Recent benhami in Zelippistes, but Dr. Marwick has pointed out to me that relationship is rather with postulatus; I have since seen the unique holotype and quite agree. It comes from the Geological Survey locality 1144, Okoke Road, 60 chains west of Pehu Trig., in the upper Waitara Survey District. Grange (N.Z.G.S. Bull. No. 31, N.S., p. 37, 1927) places this locality in the Urenui beds about 300–700 ft. above the base; the Urenui and Tongaporutu beds he includes (l.c., p. 39) in a new stage, the Taranakian, between Awamoan (Lower Miocene) and Waitotaran (Lower Pliocene) Heligmope pehuensis (Marwick) seems, therefore, to be Upper Miocene, and ancestral to the Waitotaran H. postulatus. If this is so, and Heligmope does not turn up in pre-Kalimnan beds