400 PYRAMIDELLIDA. 
them. I think that im the most numerous tribes, judicious 
grouping would be more scientific than the formation of effete 
genera. : 
It is necessary to offer a remark which is applicable to all 
the Mollusca, especially to the minute ones, and peculiarly so 
to the Chemnitzie. Great care must be taken to distinguish 
between bond fide specialties, and those apparent ones brought 
on by an uneasy condition of the animal, which ought always 
to be described on the undisturbed march, when all the organs 
are naturally deployed, as at rest they are contracted; and 
violent exertion, which often arises when the animal in creep- 
ing arrives at the level of the water in the glass in which it is 
confined, or meets with an impediment, has the effect of pro- 
ducing unnatural forms: for example, the foot is often made 
to appear deeply emarginate or hollowed out by the excessive 
protrusion of the auricles, and the termination of the rostrum 
is in like manner distorted by the right and left pomts being 
exserted preternaturally ; but all these forced positions vanish 
on the deliberate march. Inattention to these points has 
occasionally led me into errors, which will be noticed under 
their respective heads; I will not call them trifling, as 
perhaps on such the distinctness of a particular species might 
hinge. 
In further explanation of the above remarks, it is proper 
to observe, that as regards the generic characters of the 
tentacula, we have only given the usual undisturbed aspect ; 
but when the animal is disquieted, it effects, at will, various 
changes of shape of the lateral membranes, such as folding 
them longitudinally with a slight spiral tendency, or con- 
torting them into an auriform figure. These phases, in quiet 
progression, in a great measure disappear, and the tenta- 
cula become smooth, triangular, pomted, bevelled, and sym- 
metrical,—even the minute apical lobes vanish; these are, 
I believe, caused by the contraction of the skin by the tenta- 
cular muscle; they vary greatly in the different species; even 
in the same, the shape and position constantly fluctuate, being 
flat, globular, elongated, often appearing in a lateral point, 
sometimes precisely central, or changing from the absolute 
