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Abstract
The valve structure of Triceratium heterogonum Barker & Meakin is basically quadrate in which two opposing angles have gently rounded apices while the other two angles are slightly more acute. The valve margin between the angular extensions is concave. Each valve has two types of processes, a single process lying within each angle, those of opposing angles identical. Lying within the broader and more rounded angle is a hyaline tubular structure, directed outwards, and terminating in a cup-like structure; at the other pole a shorter elevation terminating in a thickened hyaline rim.
A detailed description of the valve structure and processes is given, comparisons of the tubular structure made with those of other genera and a discussion on relationships with other genera, particularly that with Triceratium sensu stricto, is also given.
A new genus Petrochiscus is proposed with T. heterogonum as the generitype.