Discussions and interpretations presented in this paper are based essentially on the studies of the Lower Cretaceous sequence and marine fossil assemblages in south-eastern Arizona. Described parahoplitan fauna ( Kazanskyella, Sinzowiella ) and most of associated lamellibranchs in the lower part of the Lowell formation (Pacheta-Saavedra members) are not represented elsewhere in North America. Here belongs the oldest known species of Acila ( A. schencki ) and many other pelecypods of extra-continental relations. The earliest known Cretaceous ammonite fauna of Texas ( Dufrenoya , Travis Peak) occurs higher in the sequence (Cholla member). The next following acanthohoplitan fauna (Quajote member), exceptionally rich in species of Eurasian affinities, is again new for the United States. The strata above (Perilla member) have familiar fossils, like Trigonia stolleyi and T. mearnsi ( T. taffi auctorum in part), and together with the Mural limestone, which with strata containing Orbitolina texana and rudistid reefs rests on the Lowell formation, are equivalent to the Glen Rose of Texas. The latest Lower Cretaceous ammonite zone of Arizona (equivalent of Grayson-Del Rio of Texas), with a prolific fauna of Stoliczkaia , is present in an isolated area south of Tucson. Zonation of the revised and restricted parahoplitan and acanthohoplitan faunas, respectively below and above the Dufrenoya justinae zone, affords a good basis for approximate correlation with the Aptian of the Old World, and especially of the Caucasus. A more natural Aptian-Albian boundary, on paleontological grounds, is suggested between the Immunitoceras nolani and Hypacanthohoplites jacobi zones of the international standard Cretaceous sequence. Below the H. jacobi zone, shells with a strongly depressed or interrupted venter are not observed among adult forms of typical Parahoplitidae, whereas a marked and rapid development of species with more or less depressed venter is characteristic for that zone and the strata above. The true parahoplitan and acanthohoplitan faunas seem to terminate with the end of Aptian time (Clansayan), and apparently the natural lower boundary of the Albian is below the strata with numerous derivative parahoplitid and hoplitid species that have rapidly progressing ventral depression and interrupted ventral ribbing. In southeastern Arizona the concurrent vertical distribution of index ammonites and Trigoniae throughout the Lower Cretaceous sequence permits an interpretation of the phylogenic development of Trigoniae of the pseudo-quadratae and the v - scripta - vau groups in relation to the ammonite zones. Ontogenic studies of Trigoniae show the trend of appreciable changes in the Pseudo-Quadratae from the Kimmeridgian (Malone, Texas), through the Neocomian (Quintuco-Agrio, Argentina), to the Aptian (Lowell, Arizona). On the other hand, certain Trigoniae of the v - scripta - vau groups, with all variability of characteristics in late Jurassic and early Cretaceous species, show remarkable morphological constancy from the Kimmeridgian (Malone, Texas; Catorce, Mexico) to the Aptian (Lowell, Arizona). Paleogeographically, the Aptian (Gargasian-Clansayan) strata of southeastern Arizona are regarded as resulting from marginal incursions, with marked breaks in sedimentation and at different points of short distance from the international border, that came from a sea to the south. Periodical communication with the Aptian-Albian basin in Texas is indicated within the Arizona Cretaceous sequence by presence of the strata with Travis Peak and Glen Rose index fossils. Twenty-one new species of ammonites and twenty-two of lamellibranchs are described in the paleontological part of the paper.