Shell thick, inequilateral, inflated. Posterior end produced. Periostracum thin, adherent. Sculpture of commarginal riblets. Umbones prominent, opisthogyrate. Hinge plate wide; taxodont teeth heavy, continuous or separated by narrow subumbonal area. Ligament opisthodetic, mostly external, portion under beaks sunken. Pallial line obscure; pallial sinus unclear. Mantle lobes not fused, except in forming two small siphons. Labial palps large; palp proboscides wide. Alimentary canal with type I stomach; intestine curving to right.
A very widely distributed deep-water genus, comprising some 30 species. This genus occurs as early as the Eocene, and may date from the Jurassic. The name is from the Greek diminutive Neilos\ the river Nile; the gender is feminine.

Austrotindaria: This genus was separated from Neilonella Dall by Fleming because of the interrupted hinge line with the ligament not extending in front of the beak, and from Protonucula Cotton by the interrupted hinge line. These are slight differences but if they prove constant they should be very useful in subdividing these rather featureless shells. From what the writer has seen of deep water taxodont bivalves, details of hinge, and of chondrophore and its relation to the hinge line, and the structure of the innermost hinge teeth, are all characters that should prove of value in limiting related groups. For these reasons Austrotindaria is used to cover those New Zealand species which agree with A. wrighti at least until such time as the Nuculanid and Malletiid groups are revised on a world-wide basis.