This paper deals with an analysis of a single specimen of Tentaculites crotalinus SALTER emend. CIGUEL et al. (1984) that present a possible partial preservation of the shell. It comes from an outcrop of the Ponta Grossa Formation (Devonian) near Jaguariaiva, State of Paraná. The skeletal microstructure of the Tentaculitoidea shell is rather poorly known. Thus, the phylogenetic relationships of these invertebrates is still uncertain. Tentaculitids are very common fossils in the Devonian Ponta Grossa Formation (Parana Basin) but so far are known only from external and internal moulds. The objective of this study is to demonstrate whether the layer found in between the internal and external mould is or not a case of shell preservation. The chemical composition of this layer and of the external mould was analysed by X rays. X rays difratometry was applied only to the matrix (at the external mould). The structure of the supposed shell remain was studied by binocular stereomicroscope and scanning electron microscope. It revealed a foliate structure due to several paralel to subparalel oriented lamelae, with a chemical composition distinct from the matrix, and some elements possibly mantained from the original shell composition. Thus, a partial preservation of the original shell seems to be the better explanation for this case. This shell would be composed by calcium carbonate disposed in laminar layers, the outer ones being impregnated by Fe and Mn molecules during the diagenetic processes. The skeletal composition and microstructure would suggest a molluscan affinity for the Tentaculitida Order
