Discussion

Of the modern struthiolariids, Tylospira would seem to be the closest to Pelicaria in the characters of the animal, radula and operculum, as well as in the scaphelloid apex and the probable loss of the free-swimming larval stage. Both are also close—though rather less so—to Struthiolaria, and in the more fundamental characters of the animal have shown great conservatism to change. Pelicaria is of rather late (Upper Miocene) New Zealand origin, and its resemblances to the Australian Tylospira therefore involve the same difficulties as Marwick (1952) has pointed to, in discussing the Pelicaria-Singletonaria relationship. Tylospira must have been restricted in its range by the loss of the free-swimming veliger stage, and—rather than to postulate a post-Miocene crossing of the Tasman—it seems much more reasonable on geographical grounds to suppose that Tylospira took its origin from Struthiolaria in New Zealand at a much earlier date, possibly in the Oligocene. As has been already pointed out (Morton, 1950), it is not necessary to insist that the scaphelloid apex was developed only once in the Struthiolariidae. Its occurrence is a normal corollary of the incubation of the eggs in a brood pouch, with the consequent need of a larger volume of yolk; and it is very possible that parallel evolution of incubation has taken place in the two most advanced genera of the family—namely, Pelicaria and Tylospira. In the same manner, the evolution of the radula in these genera, with its elongated central cusps and loss of denticles on the marginals, might be