Neilo rugata: This species appears to be a living form of the Tertiary line represented by sublaevis Marwick (Taranakian) and annectens Powell (Waitotaran) in which the sculpture consists of growth folds rather than definite raised ridges or lamellae. It differs markedly from Neilo australis with which it occurs, and which is sculptured with sharp, distant, thin ridges. It also lacks the projecting upper rostrum of this species. It agrees with annectens Powell in general characters, but the postero-dorsal extremity is rounded, the dorsal margins fall away from the beaks much more markedly and the anterior margin slopes much more rapidly. In addition the folds are wider spaced (about 3 per mm.) and more strongly developed. There are minor, fine, radially divaricating folds developed especially over the posterior surface. Hinge normal, with 18 teeth in the anterior series, and 30 in the posterior. Length, 34,0 mm.; height. 19,5 mm.; inflation (single valve). 6,7 mm.
This series shows a progression from sublaevis to rugata in the development of the strength of the concentric sculpture. N. australis also occurs in deep water and was obtained in the same general area.