characteristic of the cement-ducts in all the genera, excepting those already described, which are allied to Coronula. _Chelonobia patula._--The cementing apparatus is here chiefly remarkable for the thinness and straightness of the main trunk, (_f f_, Pl. 28, fig. 2), and from the great distance at which the glands stand apart; had another gland been drawn, it would, on the scale here used, have stood exactly under the two upper, (_c′ c′_) in fig. 1 _c_. We here see that the trunk (_f_), before entering the gland (_h_), has an enlarged portion (_g_); this, I suspect, is a very general structure. Each gland gives out, on opposite sides, two ducts (_a a_, _b b_), larger even than the main trunk; and these ducts bifurcate repeatedly, and inosculate. By this inosculation it is not improbable that all four ducts, proceeding from the two glands of the same age, may be connected together; certainly the bifurcating branches from the same duct thus become repeatedly connected. For the first two or three bifurcations the ducts decrease very little or not at all in diameter; but nearer the circumference they become smaller. The ducts, also, proceeding from the younger and smaller glands, are, of course, proportionably smaller. In one case I was able to count four bifurcations in the duct between the gland and the edge of the basal membrane. It follows from this structure, that the basal membrane, at each period of growth, is cemented down by cement issuing from several orifices; but we shall presently find that in other genera the cement proceeds from many more orifices. In fig. 2 there is represented, by the aid of the camera, a small portion (from the outer (_a_) to the outer (_b_) being 12/100ths of an inch in length) of the basal membrane, with all the several cement-ducts adhering to it, which I could distinguish, and drawn of their proper relative sizes; this figure also shows some of the bifurcations, but no inosculation happened to be included in the space here given; the basal membrane itself has not been represented. In taking a view of a considerable portion of the basal membrane, especially towards the circumference, some parallelism in the branches could be perceived; one set of branches tending to run in the direction of the ray of the circle, and the other set in the line of the circumference.