D.—ZOOLOGY 69 
are frequently mixed in collections because of their general likeness. They 
differ slightly in general outline and unguicularis is very slightly more 
brassy in colour than affinis, but otherwise only the male secondary 
sexual characters will separate the species. ‘The males differ in the shape 
of the tooth on the inner front claw, in the shape of the edeagus and in 
the form of the stridulatory file on the under side of the abdomen. Affinis 
is a peat bog species, although in areas where it does not occur, it may 
be replaced by unguicularis, the two not usually occurring together. 
Agabus nebulosus and conspersus are also found mixed in collections, as 
they closely resemble one another in general appearance. The former 
inhabits freshwater ponds and the latter brackish pools, but either can 
live in fresh or brackish water. Nebulosus is usually identified by inex- 
perienced amateurs by two black spots on the pronotum, but in brackish 
water these spots are absent. ‘The secondary sexual characters are again 
_ the best means of distinguishing the species. In both Grand Canary 
and Madeira, nebulosus occurs and, even under microscope examination, 
it is impossible to find any difference between these ‘ Atlantic’ specimens 
and those from Europe and North Africa. In the Azores, however, is 
A. godmanni, which is barely distinguishable from conspersus. Agabus 
bipustulatus is a species widely spread over Europe and North Africa and 
is one of the commonest British water-beetles. In the mountains of 
Scotland and on the Continent there is a narrow form of it which was given 
specific rank under the name solieri, but which differs in no way except 
shape and size from the type. In Madeira is an Agabus which in general 
_ appearance is a large bipustulatus and was so named by Wollaston. 
Sharp, however, recognised small distinctions which definitely give it 
specific rank, wollastonz, but it is difficult to believe that it has not originated 
from bipustulatus. Agabus guttatus and biguttaius form another pair of 
closely related species, scarcely distinguishable except by the fact that 
the male biguttatus has a toothed claw on the front feet while guttatus 
has not. Both occur in Britain but guttatus extends over Scotland, while 
biguttatus only reaches northern England. In Europe, guttatus is more 
northern and diguttatus more southern and extends to the Canaries where 
_ it is rather larger than elsewhere. In Madeira there is a small species, 
_ maderensis, which appears to be a modified biguttatus, the tooth on the 
male claw showing various stages of disappearance. In Majorca biguttatus 
is represented by a form which has been given specific rank under the 
name binotatus, but the relationship with biguttatus is exceedingly close. 
Ilybius fuliginosus, a common European species, is overlapped in the south * 
and in the Mediterranean islands by meridionalis, but here again the 
differences seem to be too slight to justify specific separation. 
In all these examples, and there are others in the Dytiscidze and more 
in the Hydrophilide, there seems to be no interbreeding between the 
related forms, although in many of them opportunities for crossing must 
occur ; but no intermediates have been recognised. ‘There are, however, 
some of these species clusters which, if they could be satisfactorily 
explained, might prove to be intermediates between biological races and 
species. These are cases of which I have elsewhere called ‘ composite 
