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Show 1892.] FROM THE OLIGOCENE OF ITALY. 81 apparently degenerate modification of the brachydont selenodont molars of certain extinct Artiodactyle Ungulates. W e may, however, go a step further than this. It is, I believe, an attribute of all Artiodactyle Ungulates, whether their cheek-teeth be of the bunodont or of the selenodont type, that while the last upper deciduous or milk-molar resembles the permanent molars in form, the penultimate milk-molar is an elongated tooth of a more complex type than either the last milk-molar or the two succeeding permanent molars. In the adult dentition, on the other hand, the last upper premolar of Artiodactyles is nearly always simpler than the first molar, and in none is it more complex or longer. Now the two teeth of the specimen under consideration present precisely the same structural relationship to one another as is presented by the penultimate and last (third and fourth) upper milk-molars of Artiodactyles. I take it, therefore, not only that these teeth are third and fourth upper milk-molars, but likewise that they belonged to an animal showing decided indications of Artiodactyle affinities-these affinities being with an Artiodactyle that had assumed selenodont molars more or less' closely approaching the Merycopotamus type. This being so, the question arises whether the specimen under consideration may not have belonged to an actual Artiodactyle. To this it may be replied that, so far as I am aware, no Artiodactyle has hitherto been described possessing molar teeth of the type under consideration ; so that if the specimen were really Artiodactylate, it would indicate an entirely new form. Apart, however, from this, the structure of the second tooth in the specimen presents such a marked Sirenian facies that, as I have said, one is prompted to at once refer the specimen to that group of mammals. If, moreover, it be compared with Baron Zigno's figure of the upper molars of Halitherium veronense, it will be seen that the last milk-molar of the present specimen accords in almost all respects with these teeth. Both have two interrupted transverse ridges, with a large anterior and a small posterior talon; and in both there is a tubercle a little to the inner side of the middle of the transverse valley. Moreover, both teeth agree in the shape of the crown; while the excess in the size of the teeth of Baron Zigno's specimen over those of the one under consideration is precisely such as we should expect to find between the milk and permanent molars of one and the same animal. It is true, indeed, that in Zigno's figure what I may call the masked selenodonti.sm of the teeth under consideration is not apparent. This may, hdwever, be due to the circumstance that the teeth of the type of H. veronense are considerably worn; while it may also be in part owing to the difficulty of expressing such features in a lithograph. Moreover, there is the possibility that the masked selenodontism of the milk-molars may not have been retained in the permanent dentition. In regard to the existence of a deciduous dentition in the Sirenia, it is already known that milk-molars and premolars were developed in Halitherium schinzi. It is, however, probable that in that species PROC. ZOOL. Soc-1892, No. VI. 6 |