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Volume 58, 1928
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A Kaipara Ammonite.

[Read before the Wellington Philosophical Institute, 7th September, 1927; received by Editor, 28th September, 1927; issued separately, 12th November, 1927.]

Plates 36, 37.

Since the paper on the Upper Cretaceous ammonites of New Zealand was written a party of geological students, working with Mr. J. A. Bartrum, Professor of Geology at the Auckland University College, collected a number of specimens. These, however, were all referable to species that were described in the paper cited above. In one case, however, a specimen was in far better condition than the type of a species described previously under the name Novakites denticulatus. Examination of this specimen shows that the species should be placed in the genus Grossouvrites established by Kilian and Reboul in 1909. A fuller decription of the species can now be given.

Grossouvrites denticulatus Marshall. (Plates).

Novakites denticulatus Marshall, Upper Cretaceous Ammonites of New Zealand, Trans. N.Z. Inst. vol. 56, 1926, p. 189, Pl. 25, fig. 3, Pl. 38, figs 5, 6.

Compare:

1895. Holcodicus gemmatus Huppé, in Steinmann Neues. Jahrb. fur min., etc. Band 10, Stuttgart, 1895.
1909. Kossmaticeras (Grossouvritos) gemmatum Huppé, sp in Kilian and Reboul, in Les Cephalopodes Nèocretacées des Iles Seymour et Snow Mill. Wiss. Ergeb. der Schwedischen sudpolar-expedition, Stockholm, 1909.
1900. Pachydiscus gemmatus Huppé, in Andersen, Graham Land, p. 35.

Dimensions (all in millimeters):—

A B
Diameter 142 100 44 100
Height of last whorl 66 46 24 54
Width of last whorl 62 44 21 48
Umbilicus 42 30 6 13.8
  • A.—Grossouvrites denticulatus, Bull's Point, Kaipara Harbour. New Zealand.

  • B.—Grossouvrites gemmatus. Measurements from cast of specimen from Seymour Island kindly presented by the late Professor Kilian.

The present specimen is far more perfect than the one described by Marshall (l.c., p. 189) under the name of Nowakites denticulatus. The description of the costation that was given requires no additions. In the present specimen, however, the suture-line both internal and

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external can be followed in some detail, except in the upper portion of the external saddle. Examination of these shows that they approach the Jacobites and Gunnarites type and are really quite distinct from those of Pachydiscus. The resemblance to that of Jacobites is most pronounced in all the main features of the external and internal portion of the suture-line, and it is to this genus that Grossouvrites has the closest relationship. On the other hand in all of the specimens that have been collected up to the present time the ornamentation does not show any approach to that of Jacobites, especially in the absence of the tendency towards the development of spinous processes on the mature whorls.

The suture-line shows a close resemblance to that of Kossmaticeras gemmatum as drawn by Steinmann and copied by Kilian and Reboul (l.c., p. 22, fig. 15). In that case, however, the whole of the external portion of the suture-line is not shown and none of the internal portion has been drawn.

The resemblance, however, is so close that it justifies the classification of this species in the genus Grossouvrites. The ornamentation, however, is different; for in Grossouvrites gemmatum half of the ribs do not extend to the umbilicus, while in this species they are all continuous. Again, the ribs of G. gemmatum are not finely denticulate as in this species. The dimensions show a general similarity to those of G. gemmatum though the differences are quite sufficient to show a specific if not a generic divergence.

The suggestion that was made in the previous paper (Marshall, l.c., p. 168) that G. gemmatum would be more properly placed in the genus Pachydiscus must now be revised. It becomes clear that Grossouvrites is quite distinct from Pachydiscus and is closely related to Gunnarites and Jacobites which may be derived from Acanthoceras on from the same stock as that genus.

In October, 1926. an opportunity was taken of comparing examples of the species of the New Zealand ammonites with specimens from the Upper Cretaceous rocks of Japan with the great advantage of consultation with Professor H. Yabe and Dr. Shimizu at the Imperial University Sendai, Japan. The opinion that was previously expressed that the New Zealand species of Gaudryceras were distinct from, though related to, those of Japan was amply verified. In other respects the opinions formed from the study of the papers written by Japanese specialists were in general strongly supported except in regard to Acanthoceras ultimum. This was found to belong to an undescribed genus of the Kossmaticeras group, and for this genus I now propose the name Aucklandites.

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Gunnarites denticulatus Marshall, two-thirds nat. size.

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Gunnarites denticulatus Marshall.
a, External suture-line; b, Internal suture-line.