900 On Aquatic Curnivorous Coleoptera or Dytisade. 
commencement in this arrangement of the species. The hind tarsi have the joints 
lobed beneath externally. but to a variable extent ; their claws are often extremely 
unequal in length, but this is not constantly so, and in Col. pacificus they are very 
nearly equal ; there is no file on the second ventral segment except in Colymbetes 
nitidus, where there is an extremely fine one ; the penultimate abdominal stigma is 
smaliin comparison with what it is in the following genus; the prothoracic side margin 
is usually quite distinct, but it is very fine in Rhantus plantaris (No. 923) whereas in 
Colymbetes capensis (No. 957),it is broad, but so obsolete that it is not easily detected, 
and in Rhantus goudoti (No. 958) and R. validus it is quite absent. The elytra are 
quite rounded at their apex. The male tersi are variable ; they always bear palettes 
beneath, which may be very minute, or comparatively large, they are more or less 
evidently laterally compressed, (very little however in R. plantaris) and the front 
claws are often very elongate, and frequently unequal. 
It will be seen that there is much variation in structure among these species, 
which to a superficial glance possess a monotorous simiarity ; a natural classifica- 
tion of the genus must be deferred till all the species are known ; the characters which 
| anticipate will prove most useful for the purpose of such grouping, are the size of 
the side-wings of the metasternum, the lobing of the hind tarsi, and the clothing 
of the male front feet, but at present no useful purpose would be served by carrying 
out such a classification. The most aberrant species are Colymbetes pacificus, (No. 
920) on account of its nearly equal hind claws; Rhantus plantaris (No. 923), on 
account of its uncompressed male tarsi, Colymbetes atricolor (No. 939), on account 
of the broader posterior portion of the epipleuree, Colymbetes nitidus, (No. 940), 
because of its possessing a fine ventral file ; Colymbetes capensis and Rhantus 
goudoti (Nos. 957 and 958), because they have the coxal processes less cleft asunder 
than usual; and these two and Rhantus validus (No. 959), because they have the 
prothoracic side margin obsolete. Dytiscus pustulatus (No. 945) may also probably 
prove to be a distinct genus. 
The genus as a whole is almost or quite cosmopolitan ; species being found even 
in the Pacific islands and New Zealand: and one of the species Colymbetes 
pulverosus (No. 924), has a remarkably wide geographical distribution in the Eastern 
hemisphere, and islands of the world, but is wanting in the New World. It is 
probable that ultimately the species of the genus will prove to be inhabitants of 
temperate regions, and when found in the tropics to be confined to a considerable 
elevation. 
I. 63.—Genus COLYMBETES. (Vide p. 624.) 
This aggregate comprises eighteen species whose individuals are of comparatively 
large size ; the wing-cases are of obscure colour with the margins more dilute and 
in nearly all the species are marked with transverse scratches or grooves, which in 
some species form a very conspicuous and beautiful sculpture. The side pieces of 
