These peculiarities extend to other classes besides fishes. Among the Bivalve shells, it has long been known that the Amazons nourishes genera of Naiades peculiar to its waters, or only found besides in the other great rivers of South America; such as Hyria, Castalia, and Mycetopus, to which I would add another genus, founded upon slender, sickle-shaped Unios, common to North and South America. But what seems to have escaped the attention of conchologists is the striking resemblance of Hyria and Avicula, of Castalia and Arca, of Mycetopus and Solen, &c. Thus exhibiting another repetition of marine types in a family exclusively limited to fresh waters, and having structural characters of its own, entirely distinct from the marine genera, the appearance of which they so closely ape. In this connection I cannot suppress the remark, that it would be puerile to consider such mimicry as indicative of a community of origin. Some of the land shells even recall marine forms; such are some of the Bulimus tribe, which resemble the genus Phasianella and Littorina far more than their own relatives. The similarity of the fringes of the anterior margin of the foot is particularly striking. The Ampullariæ remind one also, in a measure, of the marine genera Struthiolaria, Natica, &c., and many fossils of the latter family have been confounded with fresh-water Ampullariæ. The most noticeable feature of the Amazonian fauna, considered with reference to its oceanic character, is, however, the abundance of Cetaceans through its whole extent. Wherever I have navigated these waters, from Pará, where the tides still send the salt brine up the river, to Tabatinga on the borders of Peru, in all the larger and smaller tributaries of the great stream as well as in the many lakes connected with their ever-changing course, I have seen and heard them, gamboling at the surface and snoring rhythmically, when undisturbed in their breathing. At night, especially, when quietly at anchor in the river, you hardly ever fail to be startled by the noise they make, when reaching the surface to exhale forcibly the air they have long retained in their lungs while under water. I have noticed five different species of this order of animals in the waters of the Amazons, four of which belong to the family of Porpoises and one to that of Manatees. Mr. Burkhardt has drawn three of them from fresh specimens for me, and I hope before long to secure equally faithful representations of the others, when I shall describe them all comparatively. One of the Porpoises belongs to the genus Inia, and may be traced on the upper tributaries of the Amazons to Bolivia, another resembles more our common Porpoise, while still another recalls the Dolphin of the sea-coast; but I have been unable to ascertain whether any one of them is identical with the marine species. At all events,