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Show 596 MESSRS. O. THOMAS AND R. LYDEKKER ON THE [May 18, Thiers mit dem ersten eines alten, so ist dieser viel grosser als jener, so dass wenigstens eine gauze Zahnreihe gebildet und hinausgeschoben sein muss bis diese Zahne die gleiche Grosse erreicht haben." Commenting on this sentence, Lepsius \ in his classical work on Halltherium, ridicules the idea of any such continued succession of ieeth, on the ground that if true there would at least be 20 teeth in all, and states that, like other parts of the animal, the teeth increase in size as age advances. W e now know, however, that this latter statement is incorrect, and that the teeth, when once formed, do not grow at all; and this fact must be kept in mind when examining what we shall show to be the really wonderful and unique dentition of the genus Trichechus2. Our attention was first drawn to the subject by seeing the teeth of the young specimen of T. inunguis on which Mr. Beddard based the observations he read before this Society on January 19th, 18973. These teeth are so remarkably small as compared to those of adult animals, that, bearing in mind the absence in the Manatee of a vertical tooth-change, and the exceedingly gradual increase in size of the teeth as they progressively grow up behind and are thrown out in front, it becomes evident that the whole series of teeth must be very much longer than has been generally supposed. A second skull of T. inunguis slightly younger, which has been for many years in the British Museum, fully bears out this statement. (See Elate X X X V I . fig. 5.) But since T. inunguis is only represented by these two young specimens, and their comparison with adult examples of other species might readily lead to material error, we have in the succeeding part of the paper restricted ourselves entirely to the African species (T. senegalensis), of which the British Museum possesses a good series of specimens. Thus, besides a good number of more or less adult skulls, there is one very young example obtained by the late Mr. Alvan Millson at Benin, with a lower jaw 120 m m . long, a larger one from Lagos (180 mm.), and others with this measurement 200 m m . and upwards. In the youngest skull (B.M. 94. 7. 25. 8) the mandible has apparently already lost the tooth corresponding to the most anterior of the teeth in the young T. Inunguis, and has in use three teeth, respectively 8,10-2, and 10*9 m m . in length, and these would therefore be numbers 2, 3, and 4; while within the jaw 1 Abh. mittelrhein. geol. Vereins, i. p. 106 (1882). Lepsius also refers to statement by Gervais (Zool. Pal. Gen. i. p. 184, 1868) that the molars are " en nombre indetermine " ; but as Gervais goes on to say that in number they are " superieur a cinq," the statement, however true, can hardly be said to be of any importance for our present purpose. 2 Still often called Manatus, in spite of the clear proof of the incorrectness of the latter term given by many systematists and anatomists, among whom may be mentioned Wiegmann, Von Baer, Midler, Stannius, J. A. Allen, Merriam, &c. The proper scientific name of the Walrus is Odobamus. 3 P. Z. S. 1897, p. 47. |