1889.] 



On the Dentition of Ornithorhyncliiis. 



129 



On their under surfaces the teeth are rough, aud show traces of 

 the presence of minute roots, which have apparently been already 

 partly absorbed.* 



The actual dimensions of the teeth are as follows : — 



Length. Breadth. 



Upper anterior 4'0 mm. 2*3 mm. 



,, posterior 4f0 „ 2*6 „ 



Lower anterior 3"5 ,, 3*2 „ 



posterior 4*2 „ 2 - 7 „ 



Turning now to specimen b, we find that it presents a most inte- 

 resting state of affairs, although had the animal lived but a day or 

 two longer, all its interest for our present purpose would have 

 vanished, for it has all but completed the process of shedding its 

 teeth. On opening the mouth there was seen the usual set of hollows, 

 surrounded by hardened epithelium (fig. 3), characteristic of the 

 first stage in the development of the cornules. Two large hollows 

 were to be seen on each side of each jaw, and a minute additional one 

 in front above, and behind below, these hollows being of course 

 afterwards the concavities on the surface of the cornules. The 

 hollows were all filled up nearly level with the surface with bits of 

 earth and sand, and fragments of food. On cleaning this out bit by 

 bit, twof of the eight large hollows were found to contain something 

 in addition, and a close examination proved that this something was 

 in each case a worn-down tooth, reduced to about the thickness of 

 paper, and with all the outlines worn off. These remnants of teeth 

 were quite unattached, coming away freely, and would evidently very 

 soon have fallen out of their own accord. No epithelium was over 

 them, but all that surrounding and beneath them was commencing to 

 indurate and thicken, in order to form what would later have been 

 the cornule. 



These two specimens, therefore, prove the contentions put forward 

 above ; a shows that the teeth are functional, completely calcified, 

 and placed as usual close to the bone ; b that, after being worn down 

 by genuine use, they are shed from the hollows in the surface of the 

 cornules, which grow up beneath and around instead of being formed 

 above them. The specimens examined by Mr. Poulton were all from 

 animals far younger than in the case of those now described ; so far 

 younger, in fact, that instead of being at, or nearly at, their furthest 



organs in such a state of decadence as are the teeth of OrnWhorfiynclius, a view 

 that is borne out by the marked differences between the teeth now described and 

 those of the College of Surgeons specimen. — May 20, 1889. 



* This suggestion is confirmed by Professor Stewart's specimen, in which the 

 teeth have well-defined roots. — May 17, 1889. 



f The anterior right and the posterior left of the upper jaw. 



VOL. XLVI, K 



