The Natural History Museum (NHMUK) fossil reptile collections contain a set of specimens sent to Arthur Smith Woodward in 1908 by the Argentinian palaeontologist, Florentino Ameghino. This collection includes a skull and other material of Cricosaurus, a metriorhynchid thalattosuchian (or ‘sea crocodile’), a group of marine crocodylomorphs that existed from at least the Middle Jurassic to the Early Cretaceous. Handwritten labels in Spanish, probably by Ameghino, and notes in English signed by Smith Woodward are still with the specimens. Using Ameghino and Smith Woodward’s correspondence to investigate the history of the specimens, we have determined that they came from the Vaca Muerta Formation of the Neuquén Basin in Patagonia, they were in the fossil collection of the Museo Nacional, Buenos Aires (MNBA) and that Ameghino loaned them to Smith Woodward for a study that was never published. Therefore, they will be returned to Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’, Buenos Aires. These fossils, although not the most impressive, are probably the first metriorhynchid material collected in South America.