12 
CLASSIFICATION. 
its shell, aiul not fouiuled upon any particular organ, or set of organs, 
arbitrarily selected as in an artificial arrangement. 
In former times, before the structure of the Mollusca was system- 
atically e.Kamined, or its signibcance understood, they were arranged 
according to the different shapes of the shells. Ijinmi was one of 
the first to lay stress upon the structural details of the shell, noting 
the character of the umhonal teeth, ligament, folds, sculpture, etc., 
and using them to separate various groups. He also attached great 
importance to the form of the animal, and ranged all mollusks under 
five heads, as Dorh, Limax, Tethys, Sepid, and Ascidid, which 
division coincides in some measure with the classes accepted at 
the present day. 
Adanson introduced the system of classifying the bivalves in 
accordance with the muscular impressions, and took note of the 
operculum in Gastropods as an important character. 
The use of Physiological characters was first suggested by Cuvier, 
who established an arrangement based upon the pectdiarities of the 
respiratory organs, and brought together all the })ulmonate species ; 
he also proposed the still accepted terms, designating some of the 
higher groups. 
Lovfm and others have proposed the use of the lingual armature, 
as suitable organs for establishing an improved arrangement of the 
Gastropoda, and the figures that have been jniblished have often 
confirmed the truth and value of the older genera, established solely 
upon the morphology and character of the shell. 
II. von Ihering advocates the use of the nervous system in preference 
to the radula, respiratory organs, or other single character; and I 
have followed Prof. Ray Lankester and adopted this system for 
dividing two very important groups in the Gastropoda. 
All classification, however, upon whatever basis it e.xists, which 
places the objects in rotation, must be to a certain e.xtent arbitrary 
and artificial, and of necessity violate the affinities of some of the 
groups or species, because it may well be that those selected to pre- 
cede or follow any particular species or group may be no closer allied 
in general organization, than one or more others necessarily removed 
further away. If this be so, then no hard and first line can truthfully 
be drawn as to the proper secpience of many of the different groups 
or species, and we can only place the species or groups, as the case 
