400 PYKAMIDELLID^l. 



them. I think that in 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 Chemnitziae. 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 points 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, pointed, 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 



