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Show 1897.] GRINDING-TEETH OF THE MANATEE. 597 there are two more, 11*5 and 12*2 mm. long, which would be nos. 5 and 6 of the full set (Elate X X X V I . tig. 1). Comparing this with the next-sized mandible (B.M. 1388/), which shows the alveolus of one recently lost tooth, 4 teeth in place, and 2 within the jaw, w e cannot match the two posterior teeth of the youngest jaw (nos. 5 and 6) with any teeth further back in it than the 1st and 2nd of the standing teeth, respectively 10-7 and 11*3 m m . long, while they may belong still further forward, as there is a considerable difference in the ages of the two specimens. However, even this matching makes the 7 teeth of this second jaw to be numbers 4 to 10, the last having a length of 14-2 m m . (Elate X X X V I . fig. 2). The next jaw, 200 m m . long (B.M. 1388 d), gives evidence of 8 teeth, and comparing these with the last set we may match no. 10 with the fourth, so that the posterior four would be nos. 11 to 14; but it is by no means certain that the numbers should not be even higher (Elate X X X V I . fig. 3). A similar comparison with another mandible 242 m m . long (B.M. no. 94. 7. 25. 7) seems to show that the ten teeth of which evidence is shown may be nos. 11 to 20 at the lowest, a result that is fully supported by the other skulls available (Elate X X X V I . fig. 4). Any error there may be in the enumeration is on the side of making the total too low. N o essential difference appears to exist between the dentition of the upper and lower jaws, and we have therefore confined our observations to the latter as being more convenient. It is true that Dr. Kiikenthal assigns three premolars to the lower jaw and none to the upper; but all the skulls we have seen appear to have a perfectly similar dentition above and below. Among the adult skulls both sexes seem to be represented, and we have failed to find any possible cause of error in our calculations due to the factor of sex. W e have therefore, by a method which appears to be perfectly sound, arrived at a number identical with that which Lepsius considered would be the minimum outcome of Krauss's observations. But in trying to find out how many teeth a Manatee may have in its life, a further complication is introduced by the remarkable fact that in not a single specimen available to us, however large, has the growth of additional teeth behind come to an end, so that fresh teeth are apparently being produced to the close of the animal's life. It would thus seem that a long-lived Manatee might have a much larger number of teeth even than the 20 above referred to, and, in fact, if any certain method of finding out the exact number could be discovered, we should not be surprised if the total were to amount to 30 or more. But even if there are only 15 or 18 teeth to be dealt with, we are confronted with the very difficult problem of the origin and homologies of these numerous teeth, and, after that, with the bearing that their evolution has on that of other many-toothed mammals. |