372 
Corrections <fc. of an Article on Ampullaria. 
[Dec. 
Our readers may see the reason of this in the paper of MM. Dulong and Petit, pub- 
lished in the present number. We have adopted their value of the expansion of mer- 
cury as stated p. 365 to be 3 t- 5 for the centrigade thermometer which is equal to y/gt 
for every degree of Fahrenheit, or in decimals ,0001001. There is a correction 
however (if wc do not mistake the matter) which neither Mr. Daniel, nor any of 
the writers on barometric calculation have introduced into their formula;. It is the 
correction which should be made for the varying temperature of the brass scale. 
Supposing it to have been adjusted at tiO° which is I believe the temperature adher- 
ed to by English makers ; — it is evident that at a temperature of 9S° the brass scale 
has undergone a change of36°. It maybe said that this correction is small, — yet it is 
twice as great as that of glass, which Mr. Daniel lias erroneously introduced, while 
this one has been altogether neglected. The French standards are graduated at 32°. 
Here, therefore, the difference would he still greater. 
IX. Corrections of, and Additions to, an Article on Ampullaria, in 
the Second Number of the Gleanings. 
To the Editor of Gleanings in Science. 
Sir, 
I request that you will have the goodness to give an early insertion to the follow- 
ing correction of an article which appeared in the 2d. Number of the Gleanings of 
Science, for February, 1829. In that number 1 stated, that no notice on the subject 
of the genus Ampullaria had, to my knowledge, yet appeared ; whereas in the 12tli 
number of the Zoological Journal, for April, 1828, (which, from some mistake, 
only reached me on the 16th instant,) 1 find a minute and interesting account of the 
animal, from the pen of the Rev. Lansdown Guilding, of St. Vincent's ; setting my 
rough account (which was drawn up in December, 1828, from notes made in Octo- 
ber 1827) completely in the shade. It is however gratifying to me to find that, as far 
as my description goes, it is supported in its more obvious points by the more finished 
account of my fellow-labourer ; and that the Ampullaria of the East coincides with 
its brethren of the western hemisphere; the pedunculated eyes, situated at the 
exterior base of the tentacula, and the subretractile tentaculiform gente*, being 
alike in both shells. 
In the same paper Mr. Guilding has separated from the genus Ampullaria that of 
Pachystama, under which it appears that our eastern Ampullaria should be ranged. 
Deep, however, as Mr. Guilding’s knowledge of the sul ject is, 1 should hesitate to 
adopt the new genus, if on a more minute examination, and a comparison of the 
animal of our Pachystomata with Mr. Guilding ’s Ampullaria , it appears that there is 
no essential difference in the characters of the animals. In this case, Pachystama 
and Ampullaria, viewed separately, can hardly he looked upon as divisions of equal 
value with, orgven proximate importance to Puludina, which Mr. Guilding places 
as the first genus of his family of Ampullariades, and which differs so materially 
from these two genera united. The mere thickening and partial reflection of the 
peristome of the shell, and the substitution of shell for horn in the operculum, do 
not appear to me to afford sufficient generic distinction : the shell seldom exhibits 
the first character until the animal has attained its full growth, and the operculum 
is often not preserved with the shell ; so that it would be dilficult to point out the 
place of a specimen in many cases- 
In my notice on Ampullaria I mentioned that I had Paludinre with calcareous as 
well as horny opercula. A curious analogical resemblance is exhibited by the 
former to Pachystama, the peristome being thickened and snbreflccted as in that 
genus. Should the latter genus stand on the difference observable in the shell, a 
new one will be also required for these Paludinre, the animal of which, I have sa- 
tisfied myself by comparison with Paludinre with horny opercula, to be essentially 
the same. 
llie Rev. Lansdoxvn Guilding combats the opinion, that all the Trachelipoda are 
devoid ot eyes, of which some of the land P ulmonifera undoubtedly are ; and instances 
* riiese are so remarkably like tentacula, that I set them down as such in my 
Journal on my first examination of the animal at Mirzapore, in October 1826. It 
was not until 1 had an opportunity of inspecting them more leisurely, a year 
afterwards, that 1 became aware of my mistake. 
