Art. XXXIX.—Vascular System of Siphonaria obliquata Sowerby. By A. J. Cottrell, M.A., M.Sc. (N.Z.). Communicated by Professor Benham. [Read before the Otago Institute, 3rd October, 1911.] In a previous paper (Trans. N.Z. Inst., vol. 43, 1911, p. 582) I described the general anatomy of Siphonaria obliquata, the common limpet-like marine Gastropod, whose affinities are still undecided; for, while some authorities incline to the view that it is a modified Opisthobranch, others, and perhaps the majority, regard it as a Pulmonate. I now supplement that article by giving here an account of the vascular system, which was omitted purposely from that paper. The adaptation of Siphonaria to a life along the sea-shore, where at times it is below water, at other times above, is well illustrated by its “dipnoan” character, having, that is, both a lung and a gill, each with its own independent blood-supply. If, as seems the more probable, it is a Pulmonate which has left its terrestrial home and ventured back into the original habitat of the Gastropod molluscs, we must look on the gill as a new structure, analogous to those of such forms as the Prosobranch, Patella (limpets), and the Opisthobranch, Pleurophyllidia, which has arisen in relation to this new mode of life—that is, it is an “adaptive” gill, as in these forms. Its structure is not that of a typical ctenidium, as I pointed out in my previous article, though it has some resemblance to certain gills amongst the Opisthobranchs. We already know a few Pulmonates which have taken to an aquatic life in which a gill is present. Thus, in Isidora, Protancylus, and others there is a single-folded lamina; but in Siphonaria, as I have described it in my previous article (p. 585), there is a series of independent laminae arranged in a semicircle round the mantle-cavity, each lamina bearing secondary laminae, so that it is much more complex than the gill in the above Pulmonates. Lang regards these pulmonate gills as probably ctenidia, though Pelseneer and others hold them to be “adaptive” gills. On the other hand, supposing that Siphonaria is an Opisthobranch which has become able to remain out of water for a considerable portion of each day, and to breathe air during this period, we must suppose that it is the lung that is the new thing; and although, so fan as I am aware, no Opisthobranch has been found to have a lung, yet amongst the Prosobranchs several genera, belonging to different families, have developed a network of bloodvessels on the mantle-roof; so that the cavity acts as a lung, and the genera can be arranged in series, in which the ctenidium gradually decreases in size and importance till it remains as a mere vestige (as in Cerithidea). Hence, whatever view is taken as to the affinities of Siphonaria, it remains an extremely interesting form to biologists, having either an adaptive lung or an adaptive gill. The Heart. The heart lies in the pericardium on the left side of the dorsal surface of the body, just in front of the middle of the length. The position of the heart, as seen in the animal after the shell has been removed, is shown in fig. 7 of plate 28 of my former article. The walls of the pericardium