1897.] GUINDING-TEETH OF THE MATN^ATEE. 599 



Prorastomus ', again, believed to be also of Oligoeene age, whose 

 dentition has been recorded as i. |, c. ^,p. ^, m. ~, 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 Trichechits. 

 This observation we have been able to make on the type speci- 

 men of P. sirenoides from Jamaica (B.M. no. 44897), v\hich 

 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. 



From 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 the posterior molars from the germs of the anterior ones is really 

 merely due to the retardation of the grov\th 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 ^. 

 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 till it. 



We may note in this connection that Mr. M. 1\ Woodward has 

 suggested — on the jaw-lengthening theory — that the late-appearing 

 fourth molar of Gentetes is similarly a new de\ elopment ^, and 

 not a primitive character, but up to the number of four, common 

 to Ofocyon 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 Beitongia *, and even the fifth and sixth of Myrmecohius, 

 our views on the dentition of Trichechus make it clear that their 

 primitive origin must not be too confidently presumed, as has oftea 

 been the case. 



1 Owen, Quart. Journ. Geol. Soc. vol. xi. p. .541 (185.5), and xxsi. p. 559 

 (1875). See also Lydekker. Cat. Foss. Mamm. Brit. Mus. pt. v. p. 12(1887), 

 and P. Z. S. 1892, p. 77. 



^ Mr. M. P. 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. 

 • * See Thomas, Cat. Marsup. B.M. p. 105 (footnote). 



