foot of the parent muscle is partly surrounded by a portion of the oviduct, when the foot is extended in the progressive motion of the animal, that portion of the oviduct is also carried beyond the external shells, so that the young will have every facility in being set entirely at liberty. This happens, as I have mentioned, in the months of October and November, towards the end of which they have all escaped ; and even at this time young ova are found in the ovarium, preparing for the next season.

EXPLANATION OF THE PLATES.

Plate III. Fig. 1. An oyster with the convex shell uppermost, to identify the species to which it belongs.

Fig. 2. The same oyster, the convex shell being removed, showing the whole oyster, with the entire cloak, &c. &c.

Fig. 3. The same oyster, the cloak and one layer of the beard being removed, to show the ovarium and oviduct; from the edge of the ovarium is a small slice cut off, to exhibit the ova in their natural situation in an early state of pregnancy. Fig. 4. An oyster in the act of emitting its living young ones, which are enveloped in a purple coloured mucus.

N. B. All these four figures are natural size. Plate IV. Fig. 1. A perpendicular section of an oyster, to exhibit the course of the alimentary canal, the oviduct, the heart, &c. &c. the cloak and the beard being removed; magnified 2 diameters.

Fig. 2. A very small portion of the ovarium, with the ova imbedded in its substance; magnified 100 diameters.

Fig. 3. Some ova extracted from the same ovarium, as they