Text Figure A.—Radula of Monodilepas monilifera monilifera (Hutton).

Four species have previously been recognised from New Zealand waters. In the present paper an additional subspecies of monilifera is described from the Cook Strait area, and skinneri is classified as a subspecies. All the forms are closely related and from what can be seen of the Altonian representative there has been very little change since the Miocene. Slight differences are of more specific value in conservative genera, and for this reason all the New Zealand forms have not been classed as subspecies oi monilifera. Monodilepas diemenensis Finlay possesses a number of distinctive features and is therefore separated as a full species. Monodilepas otagoensis Finlay is a puzzling form in that no specimens apart from the type series appear to have been collected, and it would appear to occur within the geographical range of monilifera. Its distinguishing features are well marked, but only small shells are known. It is here accepted provisionally as a full species, at least until its adult characters, its true distribution, and its variations are better known.

One remarkable feature of the genus is that each form appears to occur in a restricted area. Monodilepas monilifera monilifera is relatively abundant in the Foveaux Strait area, with occasional sporadic occurrences to the north as far as Oamaru, M. diemenensis occurs in enormous numbers at times, near Cape Maria van Diemen, but very few specimens have been recorded to the south, the Cook Strait form appears to occur within a restricted area, while M. monilifera skinneri is, of course, confined to the Chatham Islands.

In order to make comparisons more objective, a number of measurements were made of each shell studied, and four indices were calculated from them. These measurements were total length, width, height, distance from anterior end to internal edge of foramen, and longitudinal diameter of the foramen. Where the sculpture was well enough preserved the number of radials was counted. The indices calculated were:

Width Index: The breadth of the shell expressed as a percentage of the length.

Height Index: The height expressed as a percentage of the length.

Foramen Position Index: The distance from the anterior end to the foramen expressed as a percentage of the length.

Foramen Index: The longitudinal diameter of the foramen expressed as a percentage of the length.

Monodilepas monilifera monilifera (Hutton). Text fig. B, figs. 1–3, 16–18; Pl. 16, figs. 5, 6.

Lucapina monilifera (Hutton), 1873. Cat. Mar. Moll., p. 42.

Megatebennus moniliferus (Hutton), 1893. Macleay Mem. Vol. p. 72, Pl. 8, fig. 76.

Fissuridea monilifera (Hutton); Suter, 1913. Man. N. Z. Moll., p. 105, Pl. 8, fig. 8.

Monodilepas monilifera (Hutton); Finlay, 1927. Trans. N. Z. Inst., vol. 57, p. 343.

As recognised here, this species ranges from the Snares to Cook Strait. The nominate subspecies appears to be restricted to the Forsterian. Its major characters are the attenuate anterior outline, the subtriangular muscle scar, netted sculpture with the concentric and radial ridges subequal, foramen of moderate size. The shell is fairly variable in its proportions. The number of radials varies considerably as interstitial radials are added as the shell grows. The measurements and ranges of indices give some indication of the degree of variability.

Localities. Stewart Island; Foveaux Strait; 50 fathoms off the Snares Islands; 100 fathoms off Puysegur Point, South–West Otago, 1 juvenile shell (Powell, 1927); 50 fathoms E.S.E. of Oamaru.

Fossil shells of this type have been seen from Castlecliff (Cu 3 of Fleming, 1947); muddy clay, Pukeora, Waipukurau; Okauawa Creek, Hawke's Bay, and Kai–Iwi. At first sight these fossil specimens appear separable from monilifera monilifera (Hutton) on the basis of larger size, prolongation of the anterior part of the shell, the finer sculpture and the relatively smaller foramen. Analysis of a series of living monilifera show that all these trends can be followed through the size series and that the extreme developments seen in the larger fossil shells are nothing but a consequence of larger size. Comparison of scatter diagrams comparing total length and development of the shell anterior to the foramen corroborate this point. As the shell of monilifera grows, the anterior part of the shell elongates in comparison with total length, extra radials are intercalated between the primary radials and the length of the foramen becomes shorter compared to the total length. No living shells have been seen as large as the largest fossil shells. Size differences of the order found here are not sufficient for subspecific separation. There seems, therefore, no constant feature by means of which these Pliocene shells may be separated from monilifera monilifera (Hutton).