390 Ltemarlmble Lengthening in 



object is to facilitate the student in recognising the species he 

 meets with, it can clearly hold no place in a natural one. 

 Every group in such a system must be shown to exist in 

 nature ; and every natural group -must remain entire, except 

 it contain others likewise natural and well characterised, 

 which others must yet always be considered as of mere 

 secondary importance, and as holding a subordinate rank. 

 Such a group may contain Jew or it may contain many 

 species, but its value will not be affected by circumstances of 

 this nature. As has already been observed, the value of a 

 group is determined by the importance of its characters, and 

 these are what we are to look to in the establishment of new 

 genera. If it be thought expedient, in the case of very exten- 

 sive genera, to adopt some method of abridging labour in the 

 search after particular species, we may sufficiently attain this 

 object by instituting sectional divisions. Such sections may 

 be grounded upon any character which presents itself for this 

 purpose, and be indicated by signs ; but let us not impose 

 names on collections of species which do not exist in nature 

 as distinct and well-marked groups, and raise them to a rank 

 which they manifestly do not deserve. 

 Swajfham Bulbeck, June 22. 1833. 



Art. II. A Notice of a remarkable Lengthening in the cutting 

 Teeth of the Rat, and of the physiological Principle of Dentition 

 in the Animalia rodentia generally ; tvith an incidental Notice of 

 the Dentition of the Elephant, By C 



Sir, 



In Vol. II. p. 134., Vol. III. p. 27., and Vol. VI. p. 21., 

 are given notices of a remarkable lengthening in the incisor 

 teeth of the rabbit ; but I think that not any mention has been 

 made of the same occurrence in any other species of the 

 Animalia rodentia, perhaps from your correspondents' think- 

 ing them, as the same conformation of teeth prevails through 

 the whole order, not of sufficient importance to be individually 

 noticed. 



An instance, however, of this deformity in the rat has 

 fallen under my observation ; and an account of it may be 

 interesting to some of your readers. The animal was sent 

 me by a friend, who requested me to stuff it for him ; I 

 therefore, unfortunately, was unable to dissect the head, and 

 cannot say whether the increase of growth was caused by any 

 malformation or dislocation of the jaws : the rat, however, 

 had no appearance of emaciation. My description will be 



