The two divisions which our native species of this genus form, might very consistently constitute two genera, differing so much as they do both in habit and habits. In the first section the males totally differ from their females, the latter being black and the pubescence of their partners fulvous, and whose intermediate legs are so much longer, and are decorated besides with tufts of hair upon their plantæ, neither peculiarity being found in those of the second section, which conform more regularly to the ordinary type of structure. The first section also nidificate gregariously, forming enormous colonies which consist of many hundreds; whereas the second are solitary nidificators, and at most half-a-dozen may be found within as many square yards of territory, and one species, the _A. furcata_, diverges considerably from the ordinary habits of the genus, and closely approaches those of the foreign genus _Xylocopa_, but its structure necessarily retains it within the boundaries of the genus. All these insects exhibit the peculiar characteristic of the _Scopulipedes_, in the insertion of the second joint of the posterior tarsi at the very bottom of their plantæ, conjunctively with the polliniferous scopa, placed externally upon their tibiæ and plantæ, in which characteristics the Andrenoid _Macropis_ remarkably resembles them, and which I have noticed in my remarks upon that genus. The first section burrows in banks, where their colonies are extremely numerous. In the tunnels which they form they construct several elliptical cells which they line with a delicate membrane of a white colour, formed by a secretion or saliva derived from the digestion of either the pollen or the honey which they consume. Each cell when formed is stored as usual, and the egg deposited, and then it is closed. There is but little variation in these processes among all the solitary bees, excepting in the case of the artisan bees and the more elaborate processes of _Colletes_, in which, however, the casing is merely thicker, arising from several layers of the coating membrane. The perfect insects make their appearance during the spring and summer months, their successive maturity being the result of the previous summer and autumn deposit of eggs. They pass the winter and spring in the larva state, and undergo their transformations into pupa and imago with but slight interval, and only shortly before the appearance of the perfect insect. When first presenting themselves they are certainly very handsome insects, and if carefully killed preserve their beauty for many years in the cabinet. I have found the retusa, Linn., (Kirby’s _Haworthana_,) in enormous profusion at Hampstead Heath, indeed, so numerous were they, that late in the afternoon, upon approaching the colony, they, in returning home, would strike as forcibly against me as is often done by _Melolontha vulgaris_ or _Geotrupes stercorarius_. In equal abundance I have found the _A. acervorum_ at Charlton, where I