Abstract

The lower Paleozoic fossil Ischadites is distinctly a dasycladacean alga. Its numerous branches are simple, extending from the non-calcified central axis to the periphery. Near the base of the thallus the branches are lateral and possess thick proximal calcifications, but around the apex they are radial and acicular; presumably, addition of branches was apical. Each branch bears a distal and a subdistal expansion, so that the thallus is provided with a double-walled exterior. Gametangia developed between the 2 walls. During the life of the alga they were evidently lime-encrusted to form gametocysts; they are preserved as small calcareous spheres. The presence of undoubted gametocysts in a specimen from the Trenton rocks of Michigan dispels all question of the affinities. No longer need Ischadites be placed in "Incertae Sedis" at the end of the Porifera chapter in textbooks.

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