The Cyas type of frond, except as regards the presence of a midrib in each pinna, characterizes the cycads generally, except Bowenia and Stangeria.
Such eyes on the mantle-edge are found in Pecten, Spondylus, Lima, Pinna, Pectunculus, Modiola, Cardium, Tellina, Mactra, Venus, Solen, Pholas and Galeomma.
The sporangia arise simultaneously in the sorus, which is borne on the under surface of the ordinary pinna; in those species with large sporangia the latter form a single circle, in others sporangia may also arise from the central part of the receptacle.
The development of the sporocarp shows that it corresponds to a pinna, although when mature it may appear to occupy a ventral position in relation to the vegetative portion of the leaf.
Its nearest allies are Pinna among living forms, Eligmus among fossils.
I, Part of fertile pinna, nat.