596 MESSES, o, THOMAS AND E. ltdbkkeh ON THE [May 18, 



Thiers mit dem ersten eines alien, so ist clieser viel grosser als 

 jener, so class wenigstens eine gauze Zahnreihe gebildet xmd 

 hinausgeschoben sein muss bis diese Zahne die gleiche Grosse 

 erreicht haben." 



Commenting on this sentence, Lepsius ', in his classical work on 

 Halitherium, ridicules the idea of any such continued succession 

 of teeth, 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. 



We 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 

 Trichechns ^. 



Our attention was first drawn to the subject by seeing the teeth 

 of the young specimen of T. iniinc/uis on which Mr. Beddard 

 based the observations he rend before this Society on January 19th, 

 1897'. 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 Plate XXXVI. 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 whicli 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 Millsou at Benin, with a lower 

 jaw 120 mm. long, a larger one from Lagos (180 mm.), and others 

 with this measurement 200 mm. 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 mm. in length, and these 

 would therefore be numbers 2, 3, and 4 ; while within the jaw 



^ Abh. mittelrhein. geol. Vereins, i. p. 106(1882). Lepsius also refers to the 

 statement by Gervais (Zool. Pal. Gen. i. p. 184, 1868) tbat the molars are " en 

 nombre indetermin6 " ; but as Gervais goes on to say that in number they are 

 " sup^rieur a cinq," the statement, however true, can hardly be said to be of 

 any importance for our present purpose. 



^ Still often called Manatus, in spite of the clear proof of the incorrectness of 

 the latter term given by many systematists and anatomists, among wliom may 

 be mentioned Wiegmann, Von Baer, Miiller, Stannius, J. A. Allen, Merriam, 

 &e. The proper scientific name of the Walrus is Odohanus. 



= P. Z. S. 1897, p. 47. 



