350 Mr. Jeffreys o?i the Teaiaceous Vneumonobranchous 



with many others of the land Turbines of Linnaeus, reunited 

 by Ferussac under his subgenus Cochlodina ; but, as I am 

 inclined to think, without sufficient reason. The validity 

 of a theory first proposed by our older physiological writers, 

 that a peculiarity in the form of the shell, attended by a 

 corresponding formation in its animal inhabitant, is of itself 

 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 liisus natiircp, and therefore unworthy of the attention 

 of the naturalist, or, in the prevailing rage for classification, 

 adopted as generic characters in the fullest and sometimes 

 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 Nautilida and others of the testaceous Annelides, many 

 of the individuals of which have been generically separated 

 upon no other ground than a variation in the form of their 

 shells, without any regard to the characters afforded by the 

 inhabitant. As we descend in the scale of animated nature, 



instances 



