. Bulletin of the British Museum (Natural History). 170 C. P. NUTTALL CARIBBEAN. Fig. 2 Reconstruction of early Neogene palaeogeography (See also Fig. 453, p. 351). Key: horizontal lines, ancient massifs; vertical lines, Andes mountains (except for cross-hatched, Cordillera Oriental of Colombian Andes); light stipple, maximum extent of possible brackish water basin; heavy stipple, areas where there is some fossil evidence for the existence of a brackish-water basin; drdv. drainage divides; arrows, direction of river flow; X, possible connections between brackish water basin and sea.
. Bulletin of the British Museum (Natural History). 170 C. P. NUTTALL CARIBBEAN. Fig. 2 Reconstruction of early Neogene palaeogeography (See also Fig. 453, p. 351). Key: horizontal lines, ancient massifs; vertical lines, Andes mountains (except for cross-hatched, Cordillera Oriental of Colombian Andes); light stipple, maximum extent of possible brackish water basin; heavy stipple, areas where there is some fossil evidence for the existence of a brackish-water basin; drdv. drainage divides; arrows, direction of river flow; X, possible connections between brackish water basin and sea. Numbers indicate general areas from which other non-marine faunas have been described. 1, northern Venezuela (Palmer 1945; Rutsch 1952; Macsotay 1968); 2, Middle Magdalena Valley (Pilsbry & Olsson 1935); 3, Upper Magdalena Valley (Porta 1966); 4, Cuenca and other intermontane basins of Ecuador (Bristow & Parodiz 1982). Pachydon possibly occurred as far south as the Abapo region of Bolivia (p. 344). South America is shown as an island, predating the formation of the Panama land bridge in the late Neogene. The Cordillera Oriental was raised during Middle and Late Miocene orogenies, and prior to this there would have been no barrier between the Magdalena and Amazon Valleys. During the rise of the Cordillera Oriental, the Magdalena is presumed to have formed a north-south channel whose southern end was eventually closed in the Mocoa region. Thiaridae are often surprisingly uncommon, whilst all records of the Pleuroceridae appear to be based on misidentifications of Thiaridae. As expected, Hydrobiidae are common, but the genera present in the Pebasian are largely endemic. Among the most striking features of the Pebasian is the presence of the families Neritidae, Corbulidae and Dreissenidae which are now absent from the region and tend to be indicative of at least brackish if not marginal marine conditions. Perhaps the most surprising occurrence is that of Vitrinellidae (Riss
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