two weak cinguli below, fine spirals covering the whole surface. Later in life the shell is heavily calloused (C. parasitica is completely covered), the wing is more prominent, and the columella curved. This condition shows that a gerontic stage has been reached, so that it is unlikely that Monalaria is a direct descendant of Conchothyra. The ontogeny shows, rather, that both had a common origin, but that Conchothyra became much more specialized and soon died out, while the Monalaria stock persisted.

Fig. 10. Conchothyra marshalli Trechmann (juv.); Selwyn Rapids. Compare with Monalaria minor (Plate 11, figs. 5, 6, 7).

The next development was a shortening of the axial ribs into tubercles, accompanied by evolution of the spiral sculpture along two different lines—(1) fine equal spiral lirae on a bicarinate body, (2) strong cords below a tubercled shoulder.

The former retains the straight columella and is the typical Monalaria (in which the previous stage is here included generically); but in the latter, Struthiolarella the columella becomes curved, and a considerable callus forms in some species.

In Struthiolaria s. str., which seems to date from the early Miocene or late Oligocene, there is a change in the outer lip, which becomes bisinuous, the columella is well curved, and the callus generally well developed. Sometimes it is enormously so, but these highly specialized forms did not last long. The history of the S. vermis group is somewhat uncertain; the apertural characters are the same as those of Struthiolaria s. str. (i.e., the S. papulosa group), but the ornamentation is of a very different nature, and in the course of its development shows none of the preceding stages except the first convex one. The appearance of a somewhat similar bicarination is, however, seen in young whorls of some of the Miocene Struthiolaria s. str., so it is possible that the S. vermis group diverged during early Miocene or late Oligocene times. As its appearance before the Pliocene is very brief, it is possible that the divergence was caused by isolation, which ended towards the close of the Miocene.

Tylospira, with its much-curved columella and peculiar callus, is evidently an advanced genus. The bisinuous outer lip would seem to connect it with Struthiolaria, though its early appearance in the Tertiary shows that it is not descended from that group. Perhaps both sprang from an earlier common ancestor, slightly in advance of Monalaria. A study of the ontogeny of the Australian species might throw some light on this point.

* * * * * * *

An analysis of the published lists of New Zealand Tertiary Mollusca, with a view to finding the stratigraphical range of the different species, gives a result quite disheartening to the stratigrapher. According to these lists, many species range from the bottom to the top of Oamaruian, and even into Wanganuian and Recent times. As accurate correlations with European stages or even systems cannot yet be made, such results are liable to force all New Zealand Tertiary strata into one horizon. There is already a tendency in this direction, for several geologists have put the whole of the Oamaruian into the Miocene.

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