Generic limits within the Eupodiscaceae: I. Observations on three unusual species of Cerataulus, with reference to the type species, C. turgidus

Scanning electron and light microscope observations are presented of Biddulphia brittoniana Kain & Schultze, B. heteroceros Grunow and B. multicornis Grunow. All these species possess unusual modes of chain formation, including entangling spines and clasping of the external tubes of the rimoportulae. The three species are transferred to Cerataulus Ehrenberg based on their loculate valve structure, rows of areolae with internal foramina and external groups of fine pores interrupted by spinules and locular walls, the presence of ocelli pointing away from the apical axis, and of a sipho marginalis. Observations on C. turgidus (Ehrenberg) Ehrenberg, the type species of this genus, are also provided, and the generic description is emended. Morphological features that separate Cerataulus from Pleurosira (Meneghini) Trevisan, Odontella C. Agardh and Zygoceros Ehrenberg are discussed. The overall good preservation of the materials examined here also provides new insights into the structure of the sipho marginalis.