106 Natural History of Molluscom Animals : — 



which baffle all conjecture, they begin to pass over this valley 

 every morning in a westerly direction, and return in the even- 

 ing to their eastern roosting-place in the woods of Nostell 

 Priory. 



Rooks are observed to keep up a very close and friendly 

 intercourse with starhngs [VII. 183.] and jackdaws [VI. 394. 

 516.] ; but, on looking at them in the fields, the observer will 

 perceive, that, while the jackdaws mix promiscuously with the 

 rooks, both in their flight and in searching for food, the star- 

 lings always keep in their own flock. This circumstance 

 has long engaged my attention ; but I am no farther advanced 

 in the investigation than I was on the first day on which I 

 set out. It is one of the many secrets in the habits of birds, 

 which will, perhaps, be for ever concealed from our view. 



Walton Hall, Nov. 27. 1833. Charles Waterton. 



[For remarks, by Mr. VTaterton, " on the nudity on the 

 forehead and at the base of the bill of the rook," see V. 241 

 — 245. ; and for observations, also by Mr. Waterton, " on 

 the supposed pouch under the bill of the rook," see V. 512 — 

 515. In Captain Brown's edition of W^hite's Natural History 

 of Selborne, which is noticed in VI. 133., there is a figure of 

 "a domestic hen in male plumage;" and in p. 93, 94., in a 

 long note. Captain Brown has adduced some instances of this 

 phenomenon which he had read of, or seen.] 



Art. III. An Introduction to the Natural History of Molluscous 

 Animals. In a Series of Letters. By G. J. 



Letter 12. On their Respiration. 



The respiration of the Mollusca is so slow, so little ob- 

 vious, and so easily suspended for a time, that it is possible 

 you may never have observed the process even in those 

 species which daily cross your path. You will, therefore, in 

 your next walk, please to examine the snail or the slug while 

 they are in progression, and you will see them at intervals 

 open wide a circular hole on the side of the neck and near 

 the margin of the shield or collar, and, after dilating it to the 

 utmost, they will close it again until its place becomes im- 

 perceptible ; this they do about four times in a minute, ex- 

 pelling at each time the effete air, and inhaling a fresh 

 supply. In like manner, the aquatic tribes, while crawling 

 along the surface, raise from time to time the pulmonary 

 aperture, in order to emit the vitiated air, sometimes even 

 with a crackling noise, and to receive an equal quantity 



