The arrangement of the aglaspid species in the chronological order of their stratigraphic occurrence brings out the prominence of the group in late Cambrian time and emphasizes the limited time span of known aglaspid occurrence. Both chronologically and morphologically a wide gap separates the Aglaspida from their dependent Xiphosura, Synxiphosure, and Eurypterida, while the aglaspid forebears remain to be discovered. UPPER CAMBRIAN Trempealeau formation Jordan member Aglaspis sp. Lodi member Aglaspis barrandei Hall A. spinifer nov. A. simplex nov. Aglaspella granulifera nov. A. eatoni (Whitfield) Glypharthrus thomasi (Walter) Aglaspoides sculptilis nov. A. semicircularis nov. U-arlhrus instdbilis nov. Cyclopina vulgaris nov. Chraspedops modesta nov. C. fragilis nov. Setaspis spinulosus nov. S. regularis nov. St. Lawrence member Aglaspis dorsetensis nov. Aglaspella eatoni (Whitfield) Cyclopina sp. Franconia formation Bad Axe member (and equivalent strata in Montana) Cyclopina sp. Aglaspis near franconensis nov. Aglaspella cf. eatoni (Whitfield) Hudson member Aglaspis franconensis nov. Undetermined smooth aglaspids Goodenough member Undetermined aglaspids Ironton member (and equivalent strata in Missouri and South Dakota) Glypharthrus vulpes nov. G. deadwoodensis nov. Aglaspoides ? sp. nov. Strabops thacheri Beecher Dresbach formation Crepicephalus zone Beckwithia ? major (Graham) Cedaria zone Beckwithia ? major (Graham) MIDDLE CAMBRIAN Marjum formation (highest Middle Cambrian) (from Utah) Beckwithia typa Resser STRATIGRAPHIC OCCURRENCE AND HABITAT GENERAL CONSIDERATION The limited stratigraphic range of the Aglaspida (late Middle Cambrian to late Upper Cambrian) is in marked contrast to that of the Xiphosura (Upper Devonian to Recent), more restricted than that of the Eurypterida (Ordovician to Pennsylvanian), and . . .

ABSTRACT Zone-diagnostic Middle to Upper Devonian ammonoids and conodonts occur in a sequence of thin carbonate beds in siliciclastic sections along the Allegheny front and northern fold belt of central Pennsylvania. The lowest, in the upper Tully Formation, and the succeeding four in the Harrell Shale, are southward continuations of marker beds long known in the Tully Limestone and overlying basin, slope, and shelf facies of the Genesee Group of New York State. These pelagic beds are Appalachian Basin signals of transgressive condensed intervals within global high-sea-level episodes and are associated with a worldwide biotic overturn in the tropical marine realm. The Upper Givetian Tully Pharciceras amplexum bed has close lithic and faunal equivalents in the Taghanic biocrisis interval in Morocco and southern Europe. The Lower Frasnian styliolinid upper Genundewa Limestone, with the entry of Manticoceras , and, in the succeeding dark West River Shale, the Bluff Point Bed, with the entry of late Koenenites and early palmatolepid conodonts, are Appalachian highstand signals of the worldwide Timan bioevent transgression recognized in Western Australia, the Russian Platform, southern Europe, Morocco, western Canada, and now Pennsylvania. The widespread occurrences of discrete ammonoid beds, even in the more proximal or shoreward sedimentary settings of the Catskill Delta, reinforce the view that they accumulated under conditions of sequestered sediment supply when transgressions flooded the newly vegetated and deep-root-forested delta flats. As such, they overprinted or interrupted the westward progradation of siliciclastic sediments generated by mountain-building tectophases of the Acadian orogeny. The Lower Frasnian ammonoid Epitornoceras dennisoni n. sp., described herein, from the Crosby Beds in the Harrell Shale at Milesburg, Pennsylvania, is named in honor of John Dennison.