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good method of dividing ; it may be preserved to differentiate smaller 
groups, but it seems to me very artificial and to divide the species 
into very unnatural genera. In considering the importance of the 
character it must be remembered that the ambulacra are not functional 
in any of the creatures ; the first pair of legs in all of them ha ve 
entirely ceased to be walking organs ; they are solely tactile (possibly 
also auditory), and are not ever used for walking. The same is true 
also of many other Gamasidae, and as a result the ambulacra are often 
so vestigial as to be almost obsolete ; and it is sometimes very difficult 
to say whether a species should be said to possess ambulacra or not ; 
small chitinous fixed projections sometimes take the place of the claws, 
and the distinction between one and the other is not always clear. 
With regard to the naturalness of the groups created by this mode 
of division, it seems to me that they are not happy; for instance, 
Canestrini’s genus Discopoma, which consists of creatures without 
ambulacra, contains very few species at present recorded, but amongst 
them are Kramer’s splendida and Berlese’s venusta, Acari far more 
closely allied to Uropoda carinata , U. Berlesiana, U. lamellosa, &c., 
than to cassidea and romana , which form the remainder of the genus 
Discopoma ; while the two last-named species are very similar in all 
respects, except the absence of ambulacra, to such species as TJropoda 
JRicasoIiana, U. vegetans , U. tecta , &c. 
It seems to me that the only distinction which will divide the 
Uropodinae with excavations for the legs into two really natural 
groups of closely-allied creatures is that between the species regular 
in form and with dorsal shields smooth, or simply pitted or areolated, 
and those of irregular form with dorsal shields sculptured. I admit 
that this does not sound very well on paper, but 1 do not think that 
any one who will take the trouble to arrange the creatures themselves 
into the two series by this rule will doubt that those in each group 
are closely related, and differ considerably from those in the other 
group ; if they will take the trouble to dissect them, this will become 
still more apparent. The mode of differentiation may not be quite as 
sharp and easy as a purely artificial distinction, but then a natural 
arrangement seldom is. The first group, of regular form, contains 
creatures mostly oval or round in form, with the dorsum regularly 
domed or arched, becoming quite thin at the periphery, and the dorsal 
and ventral plates quite distinct. Those of irregular form vary greatly 
in shape ; the dorsum is sculptured, mostly in a rough and irregular 
manner ; it is not regularly domed or arched ; seldom becomes thin 
at the edges, often indeed becoming thicker ; and the dorsal and 
ventral plates are generally so much fused that it is difficult to dis- 
tinguish the demarcation between them. It is necessary to find a 
name for the genus to consist of the irregular-shaped species; 
Kramer’s name of “ Trachynotus,” had it been available, would have 
been an appropriate one ; but, unluckily, that and every other really 
suitable name expressing exactly what I wish appears to be pre- 
