Seven extant and two fossil species within the family Asterolampraceae were studied with light and electron microscopy. The sequence of stages in valve morphogenesis was determined for those species where presumably developing valves were found. Occasionally valves from sibling cells were found still attached to one another, as they had been within the mother frustule during the process of cell division, giving evidence that forming, not eroding valves were being observed. Hollow rays are the most distinctive feature of genera in this family and appear to be formed by the marginal fusion of “tuning fork”-like structures emanating from a central or eccentric annulus. Rimoportulae, formerly termed labiate processes because of their lip-like internal structure, form at the marginal end of the rays where the “tines” of the forks apparently fuse. The structure of cribra, coverings over the loculate areolae, is very different in the species studied and we show the mode of development also varies.

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