This chapter is invaluable to the general student, as giving him in a single view not only a _conspectus_, of the most important attempts at classification in Zoology, but an examination of the principles involved in each, by the one among all living men most fitted to perform the task. No cultivated person who desires to know anything of Natural Science can pass over this portion of the work without careful study. Those who are not prepared to follow the author through the details of the Second Part will yet consider these volumes as indispensable companions for reference, as containing this brief but comprehensive encyclopedia and commentary, covering the whole philosophical machinery of zoological science. For the first section of this chapter Mr. Agassiz adopts the fundamental divisions (branches) of Cuvier, introducing such changes among the classes and orders as the progress of science demands. The second section gives a short account of the early attempts to classify animals, more particularly of the divisions established by Aristotle. The third section embraces the period of Linnaeus, and gives his classification. The fourth, that of Cuvier, and Anatomical systems, with the classifications of Cuvier, Lamark, De Blainville, Ehrenberg, Burmeister, Owen, Milne-Edwards, Von Siebold and Stannius, Leuckart. The fifth section includes the Physiophilosophical systems, with diagrams of Oken's and Fitzinger's classifications, and a special article for the circular groups of McLeay. The sixth and last section is devoted to Embryological systems, and presents diagrams of the classifications of Von Baer, Van Beneden, Kölliker, and Vogt. The second part of the Monograph introduces us to the consideration of a special subject of Natural History,--the North American Testudinata. Its three chapters treat successively of this order of Reptiles,--of its families,--of its North American genera and species. The THIRD PART, contained in the second volume, is entitled, "Embryology of the Turtle." It consists of two chapters: "Development of the Egg, from its first appearance to the formation of the embryo." "Development of the Embryo, from the time the egg leaves the ovary to that of the hatching of the young." Then follow the explanation of the plates and the plates themselves, thirty-four in number. We need not attempt to give any account of the parts devoted to the development of these particular subjects. This we must necessarily leave to the journals devoted to scientific matters, and the class of students most intimate with these departments of Natural Science.