Using her trowel freely, the Mason-bee has builded after the prototype of her art, the cylinder adorned with a mosaic pattern. But other cells, at least a dozen, are to follow. Necessities now obtrude themselves from which the first piece of work was exempt; that which will soon be building is subordinated to that which is already built. The solidity of the whole requires that the turrets leaning one against the other shall form a solid mass; and economy of material demands that the same partition-wall shall serve for two adjoining cells. These two conditions are incompatible with the regulation architecture, for grouped cylinders touch only along a line, affording no appreciable area of common partition-wall; they leave between them unoccupied intervals, which would prejudice the general stability. What does the builder do to remedy these two defects? She abandons the normal outline and modifies it according to the space at her disposal. She alters the shape of the cylinder, not as regards the interior, which is still kept rounded to suit the convenience of the larva, the future inhabitant, but as regards the outer envelope, which becomes irregular and polygonal, filling the interstices with its angles. The exquisite geometry promised by the turret first constructed is perforce abandoned when the complete edifice has to consist of a mass of cells in juxtaposition. Inexactness follows exactness even more noticeably at the end of the task. Anxious to strengthen her work and enable it to resist the attacks of the weather, the mason plasters it with a thick layer of mortar. Mosaic encrustations, round mouths, closed with a lid, and cylindrical bastions: all these disappear, submerged by the defensive casing. To look at, there is nothing left but a clod of dried mud. The simplest of round bodies, the cylinder, stands likewise as the model for the jam-pot wherein the Pelopæus stacks her Spiders. With mud collected from the edge of a pool, the huntress begins by building a turret ornamented with diagonal lozenges. Unhampered by its surroundings, this structure, the first of the group, is of a perfection that gives us a high opinion of the builder’s talent. It is fashioned like a segment of a twisted column. But other cells follow which, leaning one against the other, produce a mutual distortion. For the same reasons, namely, economy of material and general solidity, the beautiful ordonnance promised at the outset is wanting; crowding leads to irregularity. A thick layer of cement ends by deforming the structure altogether. Let us next consider the Agenia, who rivals the Pelopæus as a huntress and a worker in clay. She encloses the one Spider who forms her larva’s ration in an earthenware shell hardly as large as a cherry-stone and embellished on the outside with a tiny milled pattern. This little gem of ceramics is an ellipsoid truncated at one end. When the structure stands alone, its accuracy of form is perfect.