of the lumen. Towards the capsule gland its cell contents begin to change to a mucoid secretion, staining deeply purple with haematoxylin. The capsule gland itself (cp.) is a long, straight tube, with dorsoventral lumen open to the pallial cavity, with white, glandular walls, deeply furrowed by vertical rugae. The gland cells are uniformly filled with mucoid secretion, lightly staining with haematoxylin. The eggs are surrounded by a tough mucoid membrane of the same staining reaction as the capsule gland cells. Fig. 20 is of an embryo about to break through the capsule membrane. Towards the hatching stage, the embryo is equipped with a large circular operculum, with the margin unornamented. By the sharp edge of the operculum the embryo breaks through the capsule membrane. At the crawling stage depicted in Fig. 21, the embryo shell is complete, with its wide trumpet-like mouth. The foot is long and narrow, rounded behind, with finely ciliated, glandular sole, and somewhat more expanded, strongly ciliated in front. The anterior margin of the foot is incised by a groove, into which open a cluster of mucus-secreting gland cells (Fig. 21, pg.). The cephalic tentacles are long and diverging with eyes well developed at the bases. The gill filaments are narrow and club-shaped, relatively few in number, and the food groove and exhalant siphonal rudiments are already present. The shell does not become attached till a slightly later stage, and as in Serpulorbis, the crawling embryo adds a portion of the first unsculptured whorl to the completed nuclear shell.

Pyxipoma differs little from Stephopoma in the arrangement of the genital ducts. The capsule gland opens at the bottom of a deep groove, overhung by a broad fold, by which the genital duct is shut off from the strong current passing through the pallial slit. The chief feature of interest in Pyxipoma is the possession of a spacious incubatory pouch, lying within the trunk cavity. This opens forward by a wide duct with its aperture immediately beneath the right margin of the foot (Text Fig. 3, bp.) just in front of the termination of the food groove. The eggs pass forward by a ciliated furrow along the right side of the food groove, and enter the incubatory pouch, from which they emerge as creeping embryos to wander about before attachment. The brood is larger than in Stephopoma—about 100–150—and the embryos smaller, 450μ in shell diameter. The course of development is otherwise similar. The embryonic operculum is simple, flat and translucent. The development of a brood pouch with a cephalic opening, and the retention of the primitive ciliated oviducal groove, has occurred also in the freshwater mesogastropods Tanganyicia and Melania (Moore, 1899). There seems to be no point in Moore's contention that this is a highly primitive character. Ovoviviparity is always an advanced feature in mesogastropods, and incubatory devices of various types have developed separately in a number of highly specialised forms.