On account of the many exploring expeditions for which the Bay of Rio de Janeiro has been a favorite port, it has acquired a special interest for the naturalist. It may seem at first sight as if the fact that French, English, German, Russian, and American expeditions have followed each other in this locality, during the last century, each bringing away its rich harvest of specimens, by diminishing its novelty would rather lessen than increase its interest as a collecting ground. On the contrary, for the very reason that the specimens from which the greater part of the descriptions and figures contained in the published accounts of these voyages were obtained from Rio de Janeiro and its neighborhood, it becomes indispensable that every zoölogical museum aiming at scientific accuracy and completeness should have original specimens from that very locality for the identification of species already described. Otherwise doubts respecting the strict identity or specific difference of specimens obtained on other parts of the Atlantic shore, not only in South America but in Central and North America, may at any time invalidate important generalizations concerning the distribution of animals in these seas. From this point of view, the Bay of Rio de Janeiro forms a most important centre of comparison, and it was for this reason that we made so prolonged a stay there. Although the prospect of discovering any novelties was diminished by the extensive investigations of our predecessors, I well knew that whatever we collected there would greatly increase the value of our collections elsewhere. One of my special aims was to ascertain how far the marine animals inhabiting the coast of Brazil to the south of Cape Frio differed from those to the north of it, and furthermore, how the animals found along the coast between Cape Frio and Cape St. Roque differed from or agreed with those inhabiting the more northern shore of the continent and the West Indian Islands. In the course of the following chapters I shall have occasion to return, more in detail, to this subject.—L. A. Footnote 12: Molinesia and Pœcilia. Footnote 13: This lecture was accompanied by careful descriptions and drawings on the blackboard, showing the structural differences between these groups. These are omitted, as they would have little interest for the general reader. The chief object in reporting these lectures is to show the aims which Mr. Agassiz placed before himself and his companions in laying out the work of the expedition, and these are made sufficiently clear without further scientific details. Footnote 14: