The MALE differs in having the _antennæ_ longer than the thorax, filiform, but with their several joints curved, the curvature increasing towards the terminal joints, the integument of the whole of the flagellum consisting of a congeries of minute hexagons, the edges of which are all raised, and the whole resembling shagreen; the legs have the usual sexual slighter and extended development, and are necessarily less setose; it is also deficient in the transverse whitish bands of decumbent hair upon the abdomen, which is more densely pubescent on the first and second segments; and the four terminal joints of the posterior tarsi are conterminous with their plantæ. NATIVE SPECIES. 1. _longicornis_, Linnæus. 6-7 lines. (Plate VI. fig. 2 ♂♀.) _longicornis_, Kirby. GENERAL OBSERVATIONS. This genus derives its name from the great length of the antennæ in the male,—_εὖ_, _good_ or _great_, _κέρας_, _horn_. The name of the genus is usually given from some female characteristic, or from a peculiarity common to both sexes, or irrespective of any direct application, but here we find it deduced from a feature exclusively masculine. Instances of the first class we see in _Colletes_, _Halictus_, _Andrena_, _Dasypoda_, _Panurgus_, _Saropoda_, _Ceratina_, _Cœlioxys_, _Chelostoma_, _Heriades_, _Anthocopa_, and _Apathus_; of the second class we have _Prosopis_, _Sphecodes_, _Macropis_, _Anthophora_, _Nomada_, _Melecta_, perhaps _Epeolus_, according to Latreille’s idea, _Stelis_, _Anthidium_, _Osmia_, and _Bombus_; the third class comprises in our series merely _Cilissa_, and in this series the male characteristics that have suggested the name are just as few, being limited to the present genus. But the males among the bees exhibit in many cases strong and striking peculiarities which distinguish them from their partners. Exclusively of the general distinction expressed in their organic difference by the possession of one additional joint to the antennæ and one more segment to the abdomen than is exhibited in the females, we find in many cases in these two parts of their structure very marked singularities. Great sexual differences in the length of the antennæ are not restricted to the present genus; in fact, in most of the genera, this is the first striking feature, but which becomes conspicuously so in some species of _Sphecodes_, in most of the _Halicti_, in some _Nomadæ_, in _Chelostoma_, _Osmia_, _Apathus_, and _Bombus_. In _Eucera_ and _Sphecodes_, each joint of the flagellum is slightly curved, and in the former the surface of those joints appears compounded of hexagons. In _Chelostoma_ the antennæ, besides being longer than in the female, are also very much slighter and slightly compressed, and have a structure capable of curling upon itself; in the female of this genus the organ is clavate; and in _Osmia_, besides their length, in one species the male has a fringe of hair attached to one