The Silurian sponge fauna described here from Baillie-Hamilton and Cornwalls islands is one of the most diverse known from North America. The fossils are from Wenlockian-Ludlovian age deposits of the Cape Phillips Formation, which accumulated on the slope and in the basin of the Franklinian Geosyncline. The Cape Phillips Formation consists of interbedded shale, calcareous shale, and limestone, and most of the fossils are found in allochthonous limestone beds, mass movements having transported much of the fossil material downslope. Silurian sponges described here are representative of a distinct sponge biogeographic province that occurs north and northwest of a line along the Transcontinental Arch and contrasts with one to the southeast where sphaerocladine lithistid and heteractinid calcareous sponges are dominant. New genera within the demosponges include the rhizomorine Parodospongia and the orchocladines Antrospongia and Cauliculospongia. New hexactinellid genera include Corticulospongia and Lumectospongia. New species described here include the rhizomorines Haplistion frustrum and Parodospongia euhydra and the extensive orchocladines Antrospongia aberans, Aulocopium nana, Calycocoelia micropora, Cauliculospongia solida, Dunhillia fistulosa, D. megaporata, D. pluraliporosa, Patellispongia alternata, Perissocoelia (?) gelasinina, Perissocoelia (?) spinosa, Psarodictyum attenuatum, and Somersetella amplia. Astylospongiella (?) lutera and Astylospongiella striola are the only new representatives of the Sphaerocladina. The hexactinellid Amphidiscosa are represented by Lumectospongia uncinata n. gen. et sp. and the Lyssacinosa by the new Corticulospongia floccosa n. gen. et sp. and Dictyospongia apache n. sp. Carpospongia globosa (Eichwald, 1830) is reported here from North America for the first time. The broadly distributed Archaeoscyphia minganensis (Billings, 1859), Hindia sphaeroidalis (Duncan, 1879), and species of Patellispongia and Psarodictyum add "universal" elements to the assemblage.

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Moderate collections of fossil sponges have been recovered over a several-year period from a few scattered localities in west-central and east-central Alaska, and from westernmost Yukon Territory of Canada. Two fragments of the demosponge agelasiid cliefdenellid, <i>Cliefdenella alaskaensis</i> Stock, 1981, and mostly small unidentifiable additional fragments were recovered from a limestone debris flow bed in the White Mountain area, McGrath A-4 Quadrangle in west-central Alaska. Fragments of the agelasiid actinomorph girtyocoeliids <i>Girtyocoeliana epiporata</i> (Rigby & Potter, 1986) and <i>Girtyocoelia minima</i> n. sp., plus a specimen of the vaceletid colospongiid <i>Corymbospongia amplia</i> Rigby, Karl, Blodgett & Baichtal, 2005, were collected from probable Ashgillian age beds in the Livengood B-5 Quadrangle in east-central Alaska. A more extensive suite of corymbospongiids, including <i>Corymbospongia betella</i> Rigby, Potter & Blodgett, 1988, <i>C. mica</i> Rigby & Potter, 1986, and <i>C.</i>(?) <i>perforata</i> Rigby & Potter, 1986, along with the vaceletiid colospongiids <i>Pseudo-imperatoria minima</i>? (Rigby & Potter, 1986), and <i>Pseudoimperatoria media</i> (Rigby & Potter, 1986), and with the heteractinid <i>Nucha naucum</i>? Pickett & Jell, 1983, were recovered from uppermost part of the Jones Ridge Limestone (Ashgillian), on the south flank of Jones Ridge, in the Sheep Mountain Quadrangle, in westernmost Yukon Territory, Canada. The fossil sponges from the McGrath A-4 and Livengood B-5 quadrangles were recovered from attached Siberian terranes, and those from the Sheep Mountain Quadrangle were recovered from an allochthonous Laurentian terrane in the Yukon Territory

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The modest faunule of silicified fossil demosponges, documented here, was recovered from the Upper Ordovician Montgomery Limestone in the Taylorsville area, in the northern Sierra Nevada of northern California. Included are specimens of the ceractinomorph angullongiid Amblysiphonelloidea tubulara Rigby & Potter, 1986, the girtyocoelliid Girtyocoeliana epiporata (Rigby & Potter, 1986), the sebargasiid Amblysiphonella sp., and the cliefdenellids Cliefdenella alaskaensis Stock, 1981, and Rigbyetia obconica (Rigby & Potter, 1986). In addition, specimens of the vaceletiid Corymbospongia adnata Rigby & Potter, 1986, are described and figured. The assemblage is closely related to faunules of sphinctozoan sponges earlier reported by Rigby & Potter (1986) from the eastern Klamath Mountains, to the west in northern California