350 Mr. Jerrreys on the Testaceous Pneumonobranchous 
with many others of the land Turbines of Linnzeus, reunited 
by Férussac under his subgenus Cochlodina; but, as I am 
inclined to think, without sufficient reason. "The validity 1 
of a theory first proposed by our older physiological writers, i 
that a peculiarity in the form of the shell, attended by a : 
corresponding formation in its animal inhabitant, is of itself 1 
sufficient ground for systematical distinction, has been often 
questioned, but is I believe at present, with some partial 
exceptions, pretty well established. But it is most curious 
that facts, in themselves indicating the closest relation 
between the animal and its external covering, and which at 
first seem totally opposed to all the known rules of organi- 
zation, have at the same time been either disregarded as 
mere lusus nature, and therefore unworthy of the attention | 
of the naturalist, or, in the prevailing rage for classification, 3 
adopted as generic characters in the fullest and sometimes f 
most absurd extent. The reversed direction of the spire of | 
the shell in the restricted order Mollusca is, it is well known, 
influenced by the position of the circulating and respiratory 
organs of the animal; and, according to the frequency of 
its occurrence, and its presumed perpetuation in individuals, 
furnishes more or less invariable characters in the distribu- 
.. tion of. that intricate tribe. But I am convinced that the 
distinction ends here, and that it ought not to be extended 
to those tribes in which, from the more imperfect organiza- 
tion of the animals, there is not the same connexion between 
their external and internal structure. Such is the case with 
the Nautilide and others of the testaceous Annelides, many 
= E of the individuals of which have been generically separated 
upon no other ground than a variation in the form of their 
, without any regard to the characters afforded by the 
inhabitant. As we descend i in the scale of animated nature, 
instances 
