Thus _M. centuncularis_, the type of the genus, burrows in decaying wood or in brick walls, and sometimes also in the ground, and makes use of the cuttings of rose leaves,—not the petals,—and the leaves of the annual and perennial Mercury (_Mercurialis annua_ and _M. perennis_). The _M. ligniseca_ bores into sound Oak and the Mountain Ash, as well as into putrescent Elm, and uses Elm leaves to line its nests, sometimes called _centunculi_ from their being as it were patched together. This is the largest of all our species, and is found very abundantly everywhere around London frequenting the flowers of the Thistle. The _M. argentata_, Fab., or _Leachella_ of Kirby, is perhaps the prettiest of all the species, and forms its tunnels in sandbanks. I do not know what leaves this species selects, which used to be extremely rare, indeed for a long time only known by the specimen in the British Museum, until that ardent entomologist the Rev. F. W. Hope, to whom the University of Oxford owes its superb entomological collection, brought it in abundance from Southend, where, during his brief annual stay at his residence there, he used to find it in the grove which runs under the cliff edging the terrace of the village; it is extremely local, as that and Weybridge, in Surrey, are the only two spots where I have known it to be found. It is one of the most vivid fliers among the bees, and darts about, especially during brilliant sunshine in June, with the velocity of a sand-martin, and its note is shrill, but harmonious; it is not often caught upon flowers, being so extremely alert, but has been seen to visit the common Viper’s Bugloss (_Echium vulgare_). The _M. odontura_, the last of the second division, which is known only in a single male specimen in the cabinets of the British Museum, is one of Dr. Leach’s west country captures, of which nothing precise is known, and it is only noticed here on account of the singular peculiarity of the armature of the apex of its abdomen, which brings it closer to the genus _Osmia_ in that particular, although the majority of the males of the genus have the terminal segment slightly furcated. In these observations I have commenced with the division which contains the type, and to which the present name of the genus would attach from that circumstance, were it ever thought desirable to separate those species, which have dilated anterior tarsi in the males, into a distinct genus, but which I could scarcely recommend. In the arrangement of the species in the preceding list, I have placed these latter first, from their more symmetrical appearance in the cabinet, by leading down to the terminal smaller species in due order, from these larger and more conspicuous ones.