104 DR., EMBLETON OX THE TWO 



Vertebrata, 1835, can only, with tlie same exclusion, enumerate 

 sixty-one species. 



It seems probable that, at the present day, there remain un- 

 discovered few, if any, Mammalia indigenous in our island ; but 

 it may admit of question whether we have worked out a full 

 and accurate knowledge of even the sixty-one species described 

 by the latter author. 



The external characters of the animals have been repeatedly, 

 and, no doubt, correctly rendered ; it is in their internal structure 

 only that we can expect to find anything new; and our descrip- 

 tions cannot be deemed complete if the account of the organs, 

 concealed by the skin, be neglected or excluded. 



During the past summer, I have examined our two species of 

 rat, the Mus Rattus, Linn., and the Mus Decumanus, Pall., 

 having had some specimens of the former sent over by some friends 

 at Stockton, which is, as far as I know, the only locality in our 

 district where the black rat is yet to be found. A few anatomical 

 points have presented themselves, which are, perhaps, worthy of 

 being recorded; partly because the black rat is becoming rarer, 

 and possibly may, at no very distant period, become extinct in our 

 island; partly because the points alluded to show that there are 

 differences of internal structure which, independently of external 

 characters, may be grounds sufficient for establishing a specific 

 difference between the black and the brown rat ; and partly, also, 

 because I do not find these points noticed in any of the books on 

 Natural History or Comparative Anatomy to which I have access. 



Fearing to be too exclusively technical, I have prefixed to the 

 anatomical details a few words on the history of our rats, which, 

 like the dogs of the east, are the abhorred and unclean, but very 

 useful scavengers of our dirty towns. 



I will not, however, occupy your time with discussing the 

 sanitary offices performed by rats, or attempt to explain their 

 instinctive flight from the falling house or the sinking ship, or 

 notice either their wonderful migrations, or their deadly feuds and 

 massacres in times of scarcity, or their amazing fecundity, or their 

 scrupulous cleanliness whenever their indiscriminate and voracious 

 appetite is for a time appeased ; or even their tender filial affection, 



