Occasionally the length of the bottle-neck allows of a fresh arrangement and the cavity is divided into three storeys. The bottom one, which is always the most spacious, contains a female; the two above, both smaller than the first and one smaller than the other, contain males. Let us keep to the first case, which is always the most frequent. The Osmia is in the presence of one of these pear-shaped hollows. It is a find that must be employed to the best advantage: a prize of this sort is rare and falls only to fortune's favourites. To lodge two females in it at once is impossible; there is not sufficient room. To lodge two males in it would be undue generosity to a sex that is entitled to but the smallest consideration. Besides, the two sexes must be represented in almost equal numbers. The Osmia decides upon one female, whose portion shall be the better room, the lower one, which is larger, better-protected and more nicely polished, and one male, whose portion shall be the upper storey, a cramped attic, uneven and rugged in the part which encroaches on the bottle-neck. This decision is proved by numerous undeniable facts. Both Osmiae therefore can choose the sex of the egg about to be laid, seeing that they are now breaking up the laying into groups of two, a female and a male, as required by the conditions of the lodging. I have only once found Latreille's Osmia established in the nest of the Masked Anthophora. She had occupied but a small number of cells, because the others were not free, being inhabited by the Anthophora. The cells in question were divided into three storeys by partitions of green mortar; the lower storey was occupied by a female, the two others by males, with smaller cocoons. I came to an even more remarkable example. Two Anthidia of my district, A. septemdentatum, LATR., and A. bellicosum, LEP., adopt as the home of their offspring the empty shells of different snails: Helix aspersa, H. algira, H. nemoralis, H. caespitum. The first-named, the Common Snail, is the most often used, under the stone-heaps and in the crevices of old walls. Both Anthidia colonize only the second whorl of the spiral. The central part is too small and remains unoccupied. Even so with the front whorl, the largest, which is left completely empty, so much so that, on looking through the opening, it is impossible to tell whether the shell does or does not contain the Bee's nest. We have to break this last whorl if we would perceive the curious nest tucked away in the spiral.