ORDER Sirenia. The purely aquatic habits and fish-like form of the animals of this order formerly caused them to be confounded with the Cetacea, but a more intimate knowledge of their structure has shown that they really belong to a widely different type of the class. Their skeleton is remarkable for the massiveiiess and density of most of the bones of which it is composed, especially the skull and ribs, and the bodies of their vertebra? want the disc-like epiphyses so well marked in the Cetacea. The existing members of the order pass their whole life in the owater, being denizens of shallow bays, estuaries, and large rivers, but unlike the Cetacea they are never found in the high seas away from shore. Their food consists entirely of aquatic plants, either marine alga? or fresh-water grasses, on which they browse under water, as the terrestrial Ungulates do on the green pastures on land. They are generally gregarious, slow and inoffensive, and apparently s.tupid in disposition. Though occasionally found stranded by the tide or waves, there is no evidence of their voluntarily leaving the water to bask or feed on the shore.