Morphological and comparative description of the syncranium of Sotalia guianensis (P.J. van Bénéden, 1864), including ontogenic aspects and intraspecific variations are presented. The descriptions were based on the examination of 51 complete or partial skulls, kept at the Marine Mammals Lab. Collection from the Departamento de Ecologia e Zoologia, CCB, at the Federal University of Santa Catarina. The sincranium is very homogeneous and manifests its asymmetry in the morphology of the bones, nasal passages and foramina. The rostrum is proportionally narrow in relation to other Delphinidae of the same size. The temporal region is circular in young specimens, but it becomes oval in the adults were it forms a well marked temporal crest. The elliptical foramen of the periotic bone is present both in young and adult individuals. The nasals are bipartite and one may find sutural bones between the frontals. The hamular process of the pterygoids is inflated and somewhat fenestrated. The mandibular symphysis covers from 19-23% of the dentary length. Each premaxilla and maxilla has 30-35 teeth and each dentary exhibits 30-38 teeth.
anatomy; intraspecific variations; ontogeny; skull morphology; small cetaceans
