::You can read this pages to Niederberger in « Olmec Horizon Guerrero », in E. P. Benson y B. de la Fuente (eds), Olmec Art of Ancient Mexico, National Gallery of Art/H. N. Abrams, Inc., Washington/New York, pp. 95-103 and in « Mesoamerica: Genesis and First Developments », in A. H. Dani y J.-P. Mohen (eds), History of Humanity. Scientific and Cultural Development, II. From the Third Millenium to the Seventh Century BC, UNESCO/Routledge, París/Londres, pp. 462-475.