A satisfactory classification of the Naticidae is, for the following reasons, difficult to carry out: (1) The importance that has been attributed by most authors to the calcareous or horny nature of the operculum; (2) the use of the funicle in classification; (3) the absence of sculpture; (4) the great variability in shape within many of the species.

1. Cossmann (1919, p. 385) criticizes the system of generic division according to the nature of the operculum, and cites Natica dillwynni Payr. as the possessor of an operculum partly horny and partly calcareous.

2. The umbilical funicle is by no means a constant, and when coalescent with the parietal callus loses its individuality. In some cases—e.g., N. maoria—it becomes quite obsolete.

3. The only sculpture is of simple spiral grooves and cords. On Sinum and its allies this is well developed, but is of a very uniform nature throughout. In the other groups weak spirals are often present, particularly in some of the large Uber spp., but here they do not have even specific significance.

4. Dall (1892, p. 362) says, “The males, as usual, are apt to be smaller, and, not having to carry the enormous egg-sac of the females, have the ‘shoulder’ of the shell, or that part of the whorl just in front of the suture, less inflated, giving the whole shell a more evenly conical and less scalar spire. These differences are more marked in the group having a corneous operculum, but are perceptible in the others, especially those with an elevated spire. Apart from sexual differences, there is a certain variability about the coil of the shell, some specimens having a decidedly wider umbilicus than others of the same species; and the grooves and spiral ribs of the interior of the umbilicus vary within certain limits between individuals, and also have a certain range of fluctuation in the same individual at different times.”

The system of nomenclature followed below is based mainly on Dall's two papers (1892, 1909), but a departure is made in giving Amauropsella generic rank. It has also been found necessary to set up two new genera and two new subgenera. Sulconacca is proposed for some of the shells classed under Ampullina (Megatylotus) by Suter and under Lunatia by Hutton; Globisinum for the globose shells with spiral sculpture classed sometimes as Sinum and sometimes as Ampullina; Magnatica and Carinacca for Naticoid groups, the latter of which was placed under Lunatia by Hutton and Ampullina by Suter, the former under Polinices by Suter.

The table of generic and subgeneric ranges reveals no important additions to the New Zealand fauna since Bortonian times. (The one exception, Eunaticina cincta, as stated below, is based on a single specimen of doubtful authenticity.) At first sight this might seem to point to an isolation of the area during that time, preventing the arrival of new forms. Judging from our limited knowledge, however, the generic constitution of neighbouring areas does not seem to have been very different from our own. Thus new arrivals might not be noticed.