A Revision of the Upper Oxfordian Ammonites of Trept (Isère), Figured by de Riaz

Extract

In his monograph published in 1898, “Description des Ammonites des Couches à Peltoceras transversarium (Oxfordien Supérieur) de Trept (Isère),” A. de Riaz figured just under 100 ammonites, in 19 folio plates, of which 15 were devoted to the genus Perisphinctes. This work made known to geologists one of the most important Upper Oxfordian ammonite localities in the world, of special interest from its position in the Rhone valley, on the borderland between the Jura and the North-West European and Tethyan provinces. But in spite of the excellence of de Riaz’s photographs, their value is much reduced by the lack, in most cases, of either ventral views or whorl-sections, and by the failure to show septal sutures. Further, many of the photographs are more or less reduced, although stated to be of the natural size. The difficulty of identifying species from de Riaz’s work, therefore, is great; especially since for identification of members of the genus Perisphinctes a series of specimens at different growth-stages is usually indispensable.