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Summary.

A detailed account is given of the anatomy of the skeleton and dentition of Potamotherium, the first known otter, from Upper Oligocene lacustrine deposits in Allier, central France. Almost all parts of the skeleton are represented: the bones are plentiful and well preserved, displaying many aquatic adaptations.

The taxonomy of the Lutrinae, the stratigraphy of the deposits containing Potamotherium and their associated fauna are briefly discussed in the introduction.

Study of the dentition includes consideration of milk teeth. Potamotherium is the only lutrine retaining the second upper molar and the dentitions, both milk and permanent, are rather more complex than those of present day lutrines, though basically very similar to Lutra: the differences are interpreted taxonomically but no functional explanations are possible.

The description of the skull bones is outlined and the structures transmitted by the cranial foramina interpolated. Detailed study is centred on the brain, ear and nasal region. The pattern of the cortical convolutions is mapped, and the ratios of brain weight to body weight in various lutrines are compared. The petrosum reveals details of the semi-circular canals and cochlea, and two of the ossicles are described: an attempt is made to interpret auditory perception. Preservation of maxillo- and ethmo-turbinals, together with the olfactory lobes of the brain leads to an analysis of the nasal region and its functions. The mechanics of jaw movements are discussed and these considered along with the dentition suggest the likely feeding habits.

The anatomy of the vertebral column, ribs and sternum is described in detail. The main factors influencing locomotion are discussed and comparisons made with living carnivores. The poise of the neck, arching of the back and length of the tail are each considered. The varying position of the zygapo-physes are correlated with the degree of flexibility of the back and certain characters of the ribs are shown to be adaptations to aquatic breathing.

The anatomy of the pectoral girdle and fore limb includes discussion of the main muscles. The mechanical bases of the limb adaptations are explained, and these compared with fully aquatic and non-aquatic species. The rotary movements of the manus are correlated with its use in land and in water.

The study of the pelvic girdle and hind limb reveals that most bones have different proportions from those of terrestrial carnivores; this effects the mechanics of muscle actions, enabling aquatic movements to be performed more effectively. Prom the shape of the pelvic girdle the probable size of the litters is predicted. The knee joint, the patella, the muscles to the pes and the length of the metatarsals are among the features discussed in the determination of the functions of the hind limb.

In conclusion the mode of life and probable appearance of Potamotherium are considered. On land it must have closely resembled Lutra, while in the water this abundantly successful lake-dweller was a powerful swimmer.