Upper dentition of Protosciurus mengi Black, from the Orellan Brule Formation, Nebraska, is described for the first time. The zygomatic plate is tilted anterodorsally, showing the beginnings of sciuromorphy as in the Arikareean species of this genus. A new genus of sciurid, Oligospermophilus, is named for the species originally identified as ?Protosciurus douglassi (Korth), based on additional material including a skull and upper dentitions. The zygomasseteric structure of O. douglassi is unlike any previously described Tertiary sciurid. The range of O. douglassi is extended from the late Chadronian to the Whitneyan. Three distinct lineages of sciurids, distinguished on dental morphology and zygomasseteric structure, can be traced in the Oligocene in North America; they are represented by the genera Protosciurus, Oligospermophilus, and ?Protosciurus.

Baiotomeus is a new genus of ptilodontid multituberculate from the late Torrejonian Land-Mammal Age (late middle Paleocene) of western North America. Baiotomeus douglassi (Simpson), the type species, has been assigned, at one time or another, to Ptilodus, Mimetodon, and Neoplagiaulax. In addition, a second, new species, B. lamberti, from three localities in the Medicine Rocks area of southeastern Montana is reported here. The Medicine Rocks localities are tentatively assigned a latest Torrejonian age, younger than localities yielding B. douglassi and younger than previously suggested on the basis of plesiadapid primates.

Multituberculates appear to have attained their highest species richness during the Torrejonian Land-Mammal Age, but the discovery of B. lamberti illustrates that our knowledge of multituberculate diversity is incomplete from even that interval of time.

African machairodont specimens previously referred to three species of Megantereon are considered to represent a single species in turn argued to be conspecific with the Eurasian species Megantereon cultridens (Cuvier). The area of origin of Megantereon remains unclear, but doubt is expressed about claims for an earliest appearance of the genus in North America. It is probable that the North American species M. hesperus is a junior synonym of M. cultridens.

The largest known member of the mammalian order Multituberculata is the taeniolabidid Taeniolabis, which is known from Puercan (early Paleocene) localities in northern New Mexico, Utah, Montana, and southern Saskatchewan. A locality in the Ludlow Formation of southeastern Montana has produced remains of a new species, T. lamberti n. sp., which is based on a well-preserved dentary with teeth. Isolated teeth from the Tullock Formation of northeastern Montana are referred to T. lamberti n. sp. and Taeniolabis sp.

Taeniolabis triserialis, known exclusively from the San Juan Basin, New Mexico, is shown to be a junior synonym of T. taoensis. Taeniolabis sulcatus, the type species of the genus, is recognized as a nomen dubium and is synonymized with T. taoensis based on general morphology and occurrence in the same stratigraphic unit and collecting area (lower part of the Nacimiento Formation, San Juan Basin, New Mexico). The name T. taoensis is retained for this taxon in the interest of nomenclatorial stability. These synonymies limit to one the number of species of Taeniolabis recognized in the San Juan Basin, New Mexico. It is proposed that T. taoensis be designated the new type species of the genus Taeniolabis.

Piscichnus brownii n. gen. and sp. is described from the Plio-Pleistocene Koobi Fora Formation of northern Kenya. The trace fossil is an unornamented, shallow, dish-shaped structure preserved in concave epirelief on sandstones. This structure is interpreted to result from the nest-excavating activities of fish.

Perryella olsoni, a new temnospondylous amphibian from the lower Permian Wellington Formation of northern Oklahoma, is described. Features of the palatine, quadratojugal, and stapes are distinctive among described temnospondyls. The specimens share derived character states with the Dissorophoidea and Trimerorhachoidea, but cannot be assigned to either of these superfamilies as presently defined. The difficulties of temnospondyl classification in light of the confusion of primitive, juvenile, and paedomorphic features are discussed.

Three successive species of Neohipparion are recognized from the Gulf Coastal Plain: N. affine from the early Clarendonian Lapara Creek Fauna of Texas; N. trampasense from the very late Clarendonian and early Hemphillian of Florida; and N. eurystyle from the late early and late Hemphillian of Florida. Numerous specimens from the Love Site, Alachua County, Florida, as well as from other faunas of similar age from Florida, Kansas, and Nebraska (e.g., the Xmas-Kat Quarries), are referred to N. trampasense. This species is ancestral to all later species of Neohipparion, as it shares with them many advanced dental characters, but often in a rudimentary or intermediate form. Five valid species constitute the monophyletic genus Neohipparion, which ranges from the late Barstovian to the latest Hemphillian (late Miocene through earliest Pliocene) in North America. A close phylogenetic relationship between Neohipparion and “Merychippus” republicanus and/or Pseudhipparion is proposed based on similarities of cranial and dental features.

Four morphologically distinct tayassuid species are present in the marine strata of the lower Chesapeake Group in Maryland and Virginia. The oldest of these, an unnamed species, occurs in bed 2 of the Calvert Formation and is the only terrestrial mammal yet known from this unit. In the upper Calvert and lower Choptank Formations three tayassuid species resemble closely in size and morphology taxa known from the middle and upper Miocene Olcott and Valentine Formations in Nebraska. “Cynorca proterva” and “Prosthennops” xiphidonticus are present in beds 10 and 14, respectively, of the Calvert Formation. “Prosthennops” niobrarensis occurs in bed 17 of the Choptank Formation. These taxa, considered together with the other known terrestrial mammals from the Chesapeake Group, indicate the following correlations with land mammal faunas from western North America: bed 2, ?late Arikareean or early Hemingfordian; bed 10, late Hemingfordian or early Barstovian; bed 14, early late Barstovian; bed 17, late late Barstovian. These correlations are largely concordant with micropaleontogical correlations for the Chesapeake Group, but the age indicated for bed 17 is younger than an age estimate based on diatoms.

Fragments of a ceratopsian dinosaur from the Maastrichtian Hell Creek Formation, Garfield County, Montana, are unlike any known ceratopsian. Diagnostic features include a low, rounded nasal boss, an especially deep rostral-premaxillary pit, and paired choanae. The snout is elongate. The new taxon, Ugrosaurus olsoni, is referred to the Ceratopsidae.

A partial cranium and axial skeleton of an individual Symbos cavifrons were excavated in 1983 from the Mesa Verde Loess on Grass Mesa, Montezuma County, Colorado. Parts of at least 24 bones were recovered, including the first complete set of cervical vertebrae known for Symbos cavifrons. This individual, radiocarbon dated at 15,970 ± 155 yr B.P. (SI-6137), contributes to the definition of the southwestern edge of the range of the species and provides new information about the nature of the vertebral column. Pathologic constriction of the transverse canals is evident in the third and seventh cervical vertebrae. The pattern of bone distribution suggests that carnivores consumed part of this animal. The radiocarbon date also establishes the last major episode of loess deposition in the Four Corners region of the southwestern United States.

The axial column in Labidosaurus is thoroughly described. Two different vertebral “morphs” are found to occur in this genus. One type is that which is classically illustrated for Labidosaurus, while the other shows alternation in height and construction of the neural arches and neural spines. A brief description of the modifications of muscle attachments in alternating forms is outlined. Analysis shows that such a structural modification probably allowed (more) efficient dorsiflexion and lateral flexion of the vertebral column. The occurrence of alternation of neural spine and arch structure is probably a primitive characteristic, but becomes variously modified in different forms. The variability observed in the axial column suggests that Labidosaurus may have been sexually dimorphic, or that the genus may consist of two different species.