Four arrangements of primary costellae known to be present in enteletacean brachiopods are described. These arrangements are referred to as the isorthoid, rhipidomelloid, resserelloid and dicoelosioid systems. It is inferred that these systems of costellae imply the presence of similar systems of primary setae and primary pallial vascula. The influence of the inferred arrangements of primary vascula on the development of the vascular system is discussed. Some data on the distribution of the four costellar systems among enteletacean genera is presented. The resserelloid and dicoelosioid systems are known only from Resserella and Dicoelosia respectively. The rhipidomelloid system is present in Rhipidomella, Cryptothyris, Aulacella, Thiemella, Pseudodicoelosia, Dalejina, Diceromyonia and, possibly, Schizophoria. The isorthoid system is present in Isorthis, Kayserella, Phragmophora, Salopina, Dalmanella, Bancroftina, Heterorthina, Howellites and Onniella.

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