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The 38 species and varieties, 27 new ones of Fenestella, described are mainly from the Permian of the Glass Mountains, West Texas. Four are from material collected by Condra in 1937 from the lower Permian of the western Urals in Russia. Identified associated Bryozoa are also listed.

The described species of Fenestella are recognized by external and internal zooecial features, and in some consideration is given also to zoarial characters. Secondary encrustation and various appendages are described but not considered specifically diagnostic.

Variability of meshwork in a species has been determined wherever possible. Spacing of branches in the same zoarium is more irregularly varied than the spacing of dissepiments, which determines the length of fenestrules. Variability of length of fenestrules thus determined is not haphazard, but in most species is either correlated with the position of fenestrules in a zoarium or is determined by regular spacing of dissepiments at fixed intermittent shorter and longer intervals. The development of carinae and carinal nodes is of specific importance and is used in grouping of higher rank: subgeneric and next below, which is called group or section. Twelve groups or sections of the genus Fenestella are recognized and defined. Four of these are distinguished within Minilia Crockford, which is characterized by a double row of nodes, and treated as a subgenus of Fenestella. A newly introduced polyphyletic subgenus Loculiporina is characterized by superstructure similar to that in the Silurian and Devonian Loculipora. Cervella Chronic is also considered a polyphyletic subgenus of Fenestella.

The stratigraphy of the Permian of the Mid-Continent of America and of European Russia is reviewed in so far as it concerns the described Bryozoa, and the intercontinental correlation of the formations to which they belong is tabulated.

The identification and description of Permian species of Fenestella has necessitated a revision of the identical or closely related Pennsylvanian forms previously described by Ulrich, Rogers, Condra, Moore, and Elias, but no overall revision of all known Pennsylvanian species of the genus has been attempted.

Microstructure of the wall in Fenestella has been studied in polarized light on the specially prepared thin (1 to 2 μ thick) sections. A complex zoarial inner skeleton in Fenestella and other fenestrate Cryptostomata is differentiated and named the colonial plexus; it distinguishes this new order, named Fenestrata, from order Cryptostomata Vine, not Ulrich. Comparative study of the microstructure of extant and fossil Cheilostomata and Cyclostomata shows that Fenestrata are closer to the latter order in the following respect: the common bud or colonial bud of Cyclostomata, as understood by Smitt and Borg, is found to be homologous to the colonial plexus, but the latter is a more extensive and complex structure than the common bud. Laminated sclerenchyma is developed secondarily on the outer and inner sides of these homologous structures in Fenestrata and in Cyclostomata. The transverse minute spicules (so-called capillaries) in the outer sclerenchyma of Fenestrata do not compare well with either pores or pseudopores of Cyclostomata, and new evidence indicates that they are calcareous entities not openings subsequently filled by calcium carbonate. They apparently originated simultaneous with the sclerenchyma, and are interpreted as calcified bases of a simple filamentous alga, much like the similarly calcified minute epiphytic Pleurocladia lacustris of northwestern Europe.

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