Prof. Gervais in 1865 described under the name Mesosaurus tenuidens the remains of a small reptile from Griqualand, South Africa, which had been brought to France more than thirty years before. It is preserved in a slab which shows the skull and anterior portion of the skeleton. As with other vertebrate remains from South Africa, its exact geological age is unknown. The author states that the animal was a little larger than the Ocellated Lizard, and has many affinities; resembling in some characters terrestrial types of reptiles, while in other characters it approaches Simosaurians and Plesiosaurians. Its cervical vertebræ have some resemblance to those of Crocodiles. The lower jaw recalls in a general way Crocodiles and Plesiosaurs. The dorsal vertebræ are rather elongated and comparable to those of Homœosaurians and of Teleosaurs. The ribs are stronger than in any known reptile except Pachypleura, and are arranged as in that genus. They have much the same relation to the ribs of other reptiles that the ribs of Sirenians have to those of other mammals. The sternal ribs, identified by Cope, were regarded as annelid tracks. The scapular arch is compared to those of Crocodiles and Plesiosaurs; but the scapula and coracoid are anchylosed. The humerus is that of a Plesiosaur or Simosaur, with an ent-epicondylar perforation, like that seen in Varanus and certain mammals. The forearm and hand are of less aquatic type than in Plesiosaurs, and approach the terrestrial type; but the ulna, unlike that of terrestrial reptiles, has no