. Bulletin of the British Museum (Natural History), Geology. ORDOVICIAN-SILURIAN BOUNDARY IN BOLIVIA AND ARGENTINA 293 ancyrea (Eisenack), Conochitina cf. chydaea Jenkins, Desmochitina sp., Cyathochitina cf. cam- panulaeformis Eisenack, Euconochitina Tangourdeau, Rhabdochitina sp. 'A', Spathochi- tina cf. clarindoi de Costa and Sphaerochitina sp. Above the argillaceous sandstones the beds grade into medium and coarse sandstones of Wenlock and Ludlow age (Magotes Negros Formation). Baldis & Pothe de Baldis (1988, this volume) have reviewed and revised this section. The Talacasto
. Bulletin of the British Museum (Natural History), Geology. ORDOVICIAN-SILURIAN BOUNDARY IN BOLIVIA AND ARGENTINA 293 ancyrea (Eisenack), Conochitina cf. chydaea Jenkins, Desmochitina sp., Cyathochitina cf. cam- panulaeformis Eisenack, Euconochitina Tangourdeau, Rhabdochitina sp. 'A', Spathochi- tina cf. clarindoi de Costa and Sphaerochitina sp. Above the argillaceous sandstones the beds grade into medium and coarse sandstones of Wenlock and Ludlow age (Magotes Negros Formation). Baldis & Pothe de Baldis (1988, this volume) have reviewed and revised this section. The Talacasto section (Figs 1, 2) is located some 16 km WNW of Talacasto railway station and has been studied by Cuerda et al. (1982). Recent collecting by the authors yielded several hundred graptolites throughout the whole of the 3-65 m of the La Chilca Shale Formation. Collecting was done every few centimetres, as closely as the friability of the shale would allow. Several confirmatory collections were made nearby. Glyptograptus persculptus occurs com- monly, both flattened and in three dimensions, in association with equally common specimens of Climacograptus angustus Perner and in addition Pseudoclimacograptus sp. nov., Glyp- tograptus sp. (an undescribed form commonly seen in the persculptus Zone in other parts of the world), Climacograptus cf. medius Tornquist, and Climacograptus normalis Lapworth. This assemblage is taken to indicate the latest Ordovician G. persculptus Zone. At 55 cm above the base of the formation G. persculptus disappears, but the remainder of the fauna continues. Rhaphidograptus sp. at 90 cm, and G. ex gr. persculptus (late forms, smaller, and with a delayed median septum) also occur between IT m and 1-38 m, where Pseudoclima- cograptus sp. nov. is also especially abundant and dominates the fauna. The Pseudoclimaco- graptus sp. nov. is close to P. fidus and P. pictus described from the acuminatus Zone of Kazakhstan by Koren & Mikhailova (1980). From 60
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