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Show 598 MESSRS. O. THOMAS AND R. LYDEKKER ON THE [May 18, Firstly, it will not, we think, be contended by anyone, especially in face of the palseontological evidence referred to below, that the great number of the teeth of the Manatee has any direct connection with the polyphyodontism of the primitive Mammalia recognized ' by many recent authors, even though Dr. Kiikenthal, in his account of the embryonic distribution of the Manatee, says2 : "Ich nehme demnach an, dass nicht weniger als drei auf einander folgende Dentitionen sich am Aufbau dieses Backzahnes beteiligen." Apart from " pre-lacteal" and " post-permanent" teeth, in whose existence, with Messrs. Wilson and Hill, we should be glad to disbelieve, the utmost number that can be made out of the ordinary mammalian set is 12, of which 4 would be milk-molars, 4 premolars, and 4 molars. This is allowing for the possibility of the milk-molars being regularly retained and the premolars coming up behind instead of below them. Since, however, even with this rather far-fetched explanation, the numbers are still far short of the total required, we are disposed to think it unlikely, and prefer to consider only the first three or four teeth as premolars, and the rest as true molars. Whether such premolars belong to the permanent or to the milk series, we have no evidence on which to hase a suggestion. In Elephants, where the tooth-succession is somewhat similar, the corresponding teeth belong to the milk and not to the permanent series. If the presence of a specially large number of teeth in this genus bad any connection with a primitive multiplication of the sets of teeth, the ancestors of Trichechus should have possessed an equally redundant dentition, and on this point we are provided with evidence to the contrary. Eor it fortunately happens that there are fossil Sirenians so closely allied to the modern ones that we may almost treat them as if they were direct ancestors. Of these, by far the most important-because the best known- is tbe Oligocene Halltherlum, of which large numbers of specimens have been described and figured by various authors, notably Drs. Krauss 3 and Lepsius *. In this genus a careful examination of the teeth seems to show that although there was a distinct tendency towards the rapid wear and degeneration of the anterior cheek-teeth so characteristic of Trichechus, yet that the series of molars did not exceed four in number, and in any case came to an end as soon as the animal was adult. This latter point, so important for our present purpose, is clearly demonstrated by Krauss's plate vi. and Lepsius's plate x. fig. 96, where may be seen a terminal molar, considered to be m.4, fully up, beginning to be worn, and yet without any trace of a posterior tooth rising up to succeed it, as would be the case in the Manatee. 1 This, apparently with good reason, is altogether denied by the latest writers on the subject, Messrs. Wilson and Hill, Quart. Journ. Micr. Sci. 1897, p. 427 et seqq. 2 Anat. Anz. xii. p. 524 (1896). 3 N. Jahrb. Min. 1862, pp. 385-414, pis. vi. & vii. 4 Abh. mittelrhein. geol. V«reins, 1882, pp. 100-200, pis. i. to x. |