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Show 1897.] - GRINDING-TEETH OE THE MANATEE. (599 Prorastomus1, again, believed to be also of Oligocene age, whose dentition has been recorded as i. g, c. j,p. j, m. j, has-whether this dental formula is correct in details or not-quite clearly no trace of a continuous succession of teeth such as occurs in Trichechus. This observation we have been able to make on the type specimen of P. slrenoldes from Jamaica (B.M. no. 44897), which Dr. Woodward has been good enough to have further developed from the matrix with a special view to the settlement of the point under discussion. Erom these facts it results that the continuous succession of teeth in Trichechus is not a primitive character, but a new development, evolved to make up for the rapid wear and tear of the cheek-teeth which must take place in an animal living on seaweed and water weeds, and consequently having a large amount of sand mixed with its food. The evolution of these extra teeth may be supposed to have taken place by a gradual extension of the process seen in the early development of the posterior molars of ordinary mammals. There it is generally considered that the appearance of the budding-out of tbe posterior molars from the germs of the anterior ones is really merely due to the retardation of the growth of the posterior end of the dental lamina in relation to the shortness of the jaw in the young animal. Then, as the jaw lengthens, the lamina grows further backwards, the molars budding off from it in succession 2. Now there seems to be no inherent reason why, if the jaw were to go on lengthening indefinitely, the dental lamina should not also go on lengthening, and equally go on budding-out more and more molars behind. And although there is of course no indefinite lengthening of the jaw in the Manatee, the exact effect of such a lengthening, so far as the teeth are concerned, is attained by the steady progression forwards of the teeth in the jaw, which would equally leave a space behind the teeth, needing further teeth to fill it. W e may note in this connection that Mr. M . E. Woodward has suggested-on the jaw-lengthening theory-that the late-appearing fourth molar of Centetes is similarly a new development3, and not a primitive character, but up to the number of four, common to Otocyon and most Marsupials, there are not the objections to bringing in the primitive theory that are so strong in the case of the Manatee. Still, with the fourth molars of Centetes and Otocyon, the fifth of Bettongia4, and even tbe fifth and sixth of Mgrmecoblus, our views on the dentition of Trichechus make it clear that their primitive origin must not be too confidently presumed, as has often been the case. 1 Owen, Quart. Journ. Geol. Soc. vol. xi. p. 541 (1855), and xxxi. (1875). See also Lydekker, Cat. Foss. Mamm. Brit. Mus. pt. v. p. 12(1887), and P. Z. S. 1892, p. 77. 2 Mr. M. F. Woodward has been good enough to supply us with a concise account of the present state of opinion on this vexed question. 3 P. Z. S. 1896, p. 572. • 4 See Thomas, Cat. Marsup. B.M. p. 105 (footnote). |