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Four distinctive benthic communities occur in the mid‐shelf deltaic deposits of the Upper Devonian Sonyea Group: the Bellerophon Community, dominated by herbivorous gastropods and deposit‐feeding bivalves; the Prodrtctella Community, dominated by brachiopods with adaptations for life on muddy substrata; the Cypricardella Community, dominated by bivalves and brachiopods with a variety of adaptations; and the Rhipidomella Community, dominated by brachiopods adapted for life on sandy substrata. The geographic and stratigraphic occurrence of the communities suggest that their distribution was controlled by two principal sets of environmental variables: (1) onshore to offshore gradients in salinity, dissolved oxygen concentrations, and sedimentary substrata, and (2) rate of delta progradation. A change in the rate of delta build‐out during mid‐Sonyea time led to a dramatic change in onshore to offshore community patterns. The change suggests that great care must be used in constructing generalized community models for longer intervals of Paleozoic time.

Pentagonopentagonalis YELTYSHEVA, 1955 (coil. coil.), for species based on crinoid-stem parts [no type species] (ICZN pend.). FAMILY-GROUP NAMES; USE OF "NOM.

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A major obstacle to phylogenetic and systematic study of the Class Bivalvia is inadequate knowledge of hundreds of obscure and poorly defined Paleozoic genera. This study provides redescriptions and modern illustrations of the type species for all Paleozoic nuculoid bivalve genera, which make up about 10 percent of the generic names proposed for Paleozoic bivalves. An attempt has been made to include all generic names proposed before 1964 with Paleozoic type species which show any evidence of phylogenetic relationship to Recent nuculoid bivalves, as typified by the genera Nucula or Nuculana. Sixty-four generic names are treated. Eight of these names are nomenclaturally invalid (objective synonyms, nomina nuda, or otherwise unavailable names), leaving 56 valid names for Paleozoic nuculoid genera. Original type specimens of the type species were located and redescribed for 39 of these 56 names. Topotypes or other conspecific specimens were used as a basis for redescribing an additional ten type species, and the remaining seven type species, mostly rare Asiatic or eastern European forms, are redescribed from the original descriptions and illustrations. Type species are designated for the first time for 4 generic names (Bicrenuh, Ledopsis, Myoplusia, Praearca), and lectotypes are first designated for 19 generic type species. Subjective considerations, including subjective synonymies, are omitted in an attempt to provide an objective foundation of facts for all future evolutionary and systematic study of nuculoid bivalves. 1 on March 16, 2015 memoirs.gsapubs.org Downloaded from on March 16, 2015 memoirs.gsapubs.org Downloaded from on March 16, 2015 memoirs.gsapubs.org Downloaded from on March 16, 2015 memoirs.gsapubs.org Downloaded from PROCEDURES GENERA INCLUDED An attempt has been made to include all generic names of bivalves proposed before 1964 with Paleozoic type species which show any evidence of relationship to Recent nuculoid bivalves, as typified by the genera Nucula and Nuculana. Three generic names (Ekstadia, Gotodonta, and Similodonta) proposed during 1964 are also included through the courtesy of Mrs. Helen Soot-Ryen, who kindly made available the manuscript of her important study of Gotland Silurian nuculoids (Soot-Ryen, 1964).Normally, both Recent and fossil nuculoids are easily recognized by their characteristic shape and by the presence of distinctive "taxodont" dentition consisting of many small, regularly spaced teeth. Most of the type species included here show both the characteristic nuculoid shape and dentition, and the affinities of such species are relatively certain. Subjective difficulties arise when the shape of a potential nuculoid type species is atypical or when the characteristic dentition is not preserved. In such cases I have preferred to err on the side of overdocumentation by including all type species which might be reasonably inferred to have nuculoid affinities. All Paleozoic type species which preserve any evidence of taxodont dentition have been included. The convergent developm...