IX. On the skull, mandible, and milk dentition of Palæomastodon, with some remarks on the tooth change in the proboscidea in general

Abstract

The structure of the skull and mandible and of the teeth of Palæomastodon has been described in a number of papers published during the last few years, and all the information available up to the end of 1905 is summarised in the ‘Catalogue of the Tertiary Vertebrates of the Fayûm,’ published early in 1906; in that volume a list of all the other papers dealing with this subject will be found. In the spring of 1906 a further expedition to the Fayûm was undertaken, and among the specimens collected were some which add considerably to our previous knowledge of Palæomastodon, more especially of the species P. wintoni, remains of which are by far the most abundant. In the present paper it is proposed to give a description of this new material, so far as it supplements the accounts already published. The most important specimens found are: (1) the skull, mandible, atlas, anterior dorsal vertebræ, a left humerus, parts of the radius and ulna, and an imperfect tibia, all belonging to a single individual. This is one of the very few cases in which it is possible to be reasonably certain that a number of bones are really associated, since in these deposits, as a rule, the skeletons seem to have been completely scattered. The skull is complete, except that some of the dorsal surface above the level of the zygomatic arch has been worn away by drifting sand; in both skull and mandible the whole dentition is in perfect preservation. (2) The nearly complete mandible of a very young individual of P. wintoni, in which the two posterior milk-molars (m. m. 3 and 4) are in wear, while the second milk-molar (m. m. 2) is represented by its alveolus. The first true molar (m. 1) is also in wear while the crown of the uncut second molar lies in the jaw behind. The germs of the third and fourth premolars (pm. 3 and 4) are found in the jaw beneath the milk teeth which they are about to replace. The incisors are of great interest; in general form they are similar to those of the adult, but their outer edges bear a number of serrations like those described as occurring in the incisors of the imperfect mandible which was made the type of Phiomia serridens, Andrews and Beadnell. It now appears that the genus Phiomia must be abandoned, since it is founded on the lower milk-dentition of a species of Palœomastodon, as in fact has already been suggested by Schlosser. This species must have been much smaller than P. wintoni and may, perhaps, be P. parvus or P. minor, probably the latter.