66 Memoirs of the Indian Museum. [Vol. II, 
(except to some extent in some species of the genus Meter alepas) by any degeneracy of 
the cirri and mouth parts such as characterizes the Alepadinæ. 
The number and nature of the valves has formed the main consideration in all 
the systems of classification that have been worked out in detail as regards the 
Pedunculata, but the variation displayed by these structures is so great that it is 
often difficult to depend on them alone. Although, therefore, it is evident that even 
the appendages and mouth parts do not exhibit altogether constant generic and specific 
characters, they afford, considered together with the valves, most important informa- 
tion as regards the classification of the minor groups included in the Tepadidæ. 
As regards the valves it may be noted that in some {e.g./m Dichelaspis 
angulata or Conchoderma virgatum) even the number of the primary valves is not con- 
stant, while their shape, relationship to one another and degree of calcification appear, 
if only a few specimens are examined, to provide sufficient justification for the sub- 
division of the species. When larger numbers of specimens are examined, however, 
it is seen that variation in any one character is not always or even usually correlated 
with variation in any other, so that, unless an indefinite number of species, based on 
various combinations and permutations of characters, is to be recognized, many forms 
which at first appeared distinct must be grouped together under one specific name. 
The exact proportions of the various appendages, the arrangement of the hairs on 
each and the exact outline of those connected with the mouth have been regarded by 
some authors as characters so constant that they may, in the opinion of these authors, 
be considered as practically devoid of variation. In many species, however, the 
examination of a large number of specimens shows that not only (in the individuals 
examined) do these characters differ from published descriptions and figures, but also 
that they vary in different individuals captured in the same place and in identical 
conditions, although the limits of variation are sometimes narrower in their case than 
in that of the valves. For example, in three specimens of Pœcilasma minutum from 
the same specimen of PantUirus chosen at random and dissected consecutively, the anal 
appendages failed to reach the tip of the basal joint of the sixth cirri in one, greatly 
surpassed this point in the second, and just reached it in the third. In the genera 
Dichelaspis and Pœcilasma , moreover, the number of teeth is not always the same on 
the two mandibles of the same individual. 
Such difficulties arise not only as regards species but also as regards genera. 
Darwin was abundantly justified in separating the species he grouped in the genus 
Dichelaspis from those he grouped in the genus Pœcilasma , although he recognized 
that the main, if not the only, difference between the two genera consisted in the 
degeneracy of the valves in Dichelaspis. Since his time, however, several species 
have been discovered which link the two genera together in such a way that any 
separation between them must be regarded as a purely artificial one, maintained for 
the sake of convenience and not on morphological grounds. 
It is often convenient to recognize genera erected on a purely artificial basis, 
because it becomes extremely difficult to deal with long lists of species grouped under 
one generic name ; but it should be clearly understood that the division in such cases 
