The Zoologist — April, 1874. 3935 



Curious Malformation of the Incisor Teeth in the Skulls of 

 Two Rats. By Francis Hancock Balkwill, Esq. 



I SEND for your inspection two rats' skulls, in which the incisor 

 teeth, from not meeting properly, have failed to wear away their 

 antagonists, and, as these teeth continue to grow during life, the 

 result has been a very curious deformity. Similar instances have 

 been recorded of the hare, rabbit and hippopotamus. I believe, 

 in the two former animals, it has been attributed to the accident 

 of the opposing tooth having been shot away. 



The larger of the two skulls was sent to me in the flesh, directly 

 after it had been caught in a trap, in a warehouse in Cork, by the 

 late Mr. J. H. Richardson, of Newcastle. 



The cause of the deformity has evidently been a large malignant 

 tumour (cancer) in the body of the lower jaw on the left side just 

 behind the molar teeth, occupying the whole of the bone there, 

 and enlarging it so as to lengthen that side and throw the tops 

 of the lower incisor teeth a little to the right of their true position ; 

 in consequence the left upper incisor has escaped wear, and has 

 grown so that, estimating as nearly as possible where the tooth 

 originates in the bone, it has completed about three-quarters of 

 a circle, instead of only half, its normal size. 



The rat, which was the common brown, was in good condition, 

 notwithstanding that the disease had so involved and softened the 

 bones as to make it quite impossible to get a perfect skull. 



The other, which is much more unique, was also brought me 

 in the flesh, by Mr. W. C. James, one of his men having just 

 caught it in his starch manufactory. It was an old English 

 black rat, — a species which he informs me is not at all dying 

 out on the starch works, — in good health and condition, with 

 nothing that can with any certainty be pointed to as accounting 

 for its extraordinary appearance. 



From some cause, in early life, the front of the lower jaw has 

 been deflected a little to the left, so as to allow the upper and 

 lower incisors to pass each other without meeting. How old it 

 was at death I should not like to say, but it had lived long enough 

 for the right upper incisor to have made two entire circles, — that 

 is, looking at the bead in profile, the point of the tooth just reaches 

 to where the tooth commences to be formed in the jaw j and as 



