Overgrowth of One Mandible in Birds 237 



a corresponding amount of force is required to penetrate it 

 if direct violence is to be employed, and this is not possible 

 to the soft Cliona." He concludes that the sponge dissolves 

 the shell by the carbonic acid gas which it evolves. 



OVERGROWTH OF ONE MANDIBLE IN BIRDS. 



The illustration here given is copied from one published in 

 the Field, in January, 1892. It had been supplied to that 

 journal by Messrs. Henderson, of Dundee, and was given 

 as an instance of "extraordinary extension of the upper 

 mandible," which was unexplained. It was added : — 



"The cause of these abnormal growths is not always evident. It 

 is not easy to see the reason why, in the present case, where there 

 is no lateral want of symmetry, the upper mandible should not have 

 been equally worn down with the lower." 



Our interest in asking attention to this drawing at the 

 present time is in connection with the cause of the deformity. 

 A specimen has recently come into our possession so exactly 

 like that here delineated that it may be suspected that it is the 

 same. It was presented with others to the Selby Educational 

 Museum. Whether it be the same or not, we may feel con- 

 fident that the cause was identical. Now it is quite certain that 

 in the Selby specimen the tip of the lower mandible has been 

 shot away. An exceedingly small portion has been removed, but 

 sufficient to disable the beak from pecking at hard substances, 

 and also to leave the tip of the upper mandible unopposed. 

 Thus the case comes into line with many other examples of 

 overgrowth from want of wearing down by use. These are 

 seen in the case of tusks and teeth, and also in claws and 

 hoofs. We have a series of such in the Haslemere Museum. 



It is worth while to remark that the injury is not always 

 obvious at first sight. It is not so in our drawing, and it is 



