Fossils and rock samples were collected from the Snowy Range Formation at 24 sections measured in the Horseshoe Hills and Bridger Mountains of Montana and eight sections in the vicinity of Yellowstone National Park in Montana and Wyoming. Where the Snowy Range Formation is overlain by the Maywood unit (Devonian), both were measured and sampled, although the Maywood proved to be unfossiliferous. Lowermost of the three members of the Snowy Range is the Dry Creek Shale which lies conformably on the Pilgrim Limestone. It consists of about 50 feet of purplish, thin-bedded, fissile to slightly plastic shale with a few irregular beds of brownish-gray, platy, dolomitic siltstone in the lower three fourths, and some thin beds of silty limestone or limestone-pebble conglomerate in the upper quarter. No evidence is present in this area for subaerial erosion between the Pilgrim and Snowy Range Formations, although beds of siltstone in the Dry Creek may reflect uplift and erosion at the source of terrigenous sediments. The middle member is the Sage, which averages about 200 feet thick where complete. Its base is marked at most localities by a 1-20-foot bioherm of columnar algal limestone; the remainder is a fairly regular alternation of 1–2-foot beds of limestone or limestone-pebble conglomerate with 2–4-foot beds of green shale or very argillaceous greenish-gray limestone. The upper part contains beds of noncolumnar algal limestone. This member was deposited far from shore in shallow turbid water that contained abundant calcium in solution. Microcrystalline calcite ooze, along with fine fragments of fossils and pellets, collected in ripple troughs and other depressions in the mud of the sea floor. These small accumulations became sufficiently consolidated to maintain coherence when excavated by currents that swept away clay particles and concentrated the limestone “pods” into beds that later were cemented to form limestone-pebble conglomerate. The uppermost member, the Grove Creek, is nowhere complete in this area; the upper part was removed by erosion at some localities and by faulting at others. The member consists of about 25 feet of dolomitized limestone-pebble or cobble conglomerate with intercalated beds of dolomitized, gray, splintery shale. This member owes much of its distinctive character to weathering that took place before deposition of the Upper Ordovician Bighorn Formation. The Maywood unit is a Devonian soil or weathered zone that overlies some part of the Snowy Range Formation in the northwestern part of the area of study but overlies formations as low as those in the Precambrian Belt Series or as high as the Upper Ordovician Bighorn Formation in other parts of Montana. It consists of a greatly varying thickness (averaging about 55 feet) of reddish-orange or brown, thin-bedded, silty dolomite or dolomitic limestone. At a few localities it is coarse-grained, thick-bedded, light-brown dolomite. No fossils were found in the Maywood of the area studied, but Devonian fossils have been collected from it in other parts of Montana. Fossils described are 80 species assigned to 48 genera of trilobites, 20 species assigned to 14 genera of brachiopods, four species in four genera of gastropods, two species in one genus of conodonts, one species of sponge, one of graptolite, two form species of algal limestone, and problematica . New taxa are the trilobites Comanchia lippa n.sp., Geragnostus ? insolitus n.sp., Monocheilus demissus n.sp., Pinctus ? artus n.sp., P. ? pullus n.sp., Pseudagnostus sentosus n.sp., Rasettia snowyensis n.sp., Saratogia carita n.sp., S. fracida n.sp., and Taenicephalus gallupensis n.sp.; the brachiopods Angulotreta catheta n.sp., A. glabra n.sp., A. vescula n.sp., Eoorthis remnicha var. A. n. var. and Huenella texana var. fortis n.var. Fossil species in the Snowy Range Formation are assigned to local zones that are based on ranges of genera and species of trilobites and which can be correlated with standard zones of the Cambrian Correlation Chart or with local zones of other areas. A few species from scattered localities near the base of the formation belong to the Aphelaspis or Dunderbergia zones of the Dresbachian Stage. Most species belong to zones in the Franconian. Lower-most of these is the Elvinia Zone (with Camaraspis Subzone and Irvingella major Zonule in the upper part) which corresponds to the Elvinia Zone of the Cambrian Correlation Chart. Next higher is the Taenicephalus Zone (with Parabolinoides Subzone at base) which correlates with the Conaspis Zone of the Chart. Above this is the Idahoia Zone (with basal I. wyomingensis Subzone, middle I. wisconsensis Subzone, and upper I. serapio Subzone) which correlates with the Ptychaspis Subzone of the Ptychaspis-Prosaukia Zone of the Chart. The uppermost zone of the Franconian is the Prosaukia Zone, which is equivalent to the Prosaukia Subzone of the Chart. Species of the Trempealeauan Stage are assigned to the Illaenurus Zone which corresponds to the Lower Trempealeauan of Texas or the lower part of the Saukia Zone of the upper Mississippi Valley. An alternative zonation is offered, based on the ranges of genera and species of brachiopods. Boundaries of most brachiopod zones fall near the levels of the boundaries of trilobite zones, but some do not. The Apsotreta expansa Zone belongs to the Dresbachian Stage. The Linnarssonella Zone and the Angulotreta tetonensis Zone (with Ceratreta-Eoorthis Subzone at base) correspond respectively to the Elvinia and Taenicephalus Zones of the lower part of the Franconian Stage. The Angulotreta vescula Zone corresponds to all but the uppermost part of the Idahoia Zone, and the A. catheta Zone (with Finkelnburgia osceola Subzone at top) is equivalent to this uppermost part of the Idahoia Zone and the entire Prosaukia Zone of the Franconian Stage as well as the entire Illaenurus Zone of the Trempealeauan Stage. No Franconian-Trempealeauan boundary is apparent in the brachiopod zonation.