_Bibliography of Sirenia._—J. F. Brandt, _Symbolæ Sirenologicæ_, St. Petersburg, 3 fasciculi, 1846-61-68—an exhaustive account of the anatomy, affinities, and literature of the group, with copious illustrations of the osteology of _Rhytina_. _Anatomy of Dugong_:—Everard Home, _Phil. Trans._ 1820, p. 315; Owen, _Proc. Zool. Soc._ 1838, p. 29. _Placenta of do._:—W. Turner, _Trans. Roy. Soc. Edin._ vol. xxxv. (1889). _Manatee_:—W. Vrolik, _Bijdragen tot de Dierkunde_, 1851; J. Murie, “On the Form and Structure of the Manatee,” _Trans. Zool. Soc. Lond._ vol. viii. p. 127, 1872, and “Further Observations on the Manatee,” _Ibid._ vol. xi. p. 19, 1880; A. H. Garrod, “Notes on the Manatee recently living in the Zoological Society’s Gardens,” _Ibid._ vol. x. p. 137, 1875; H. C. Chapman, “Observations on the Structure of the Manatee,” _Proc. Acad. Nat. Sciences of Philadelphia_, 1875, p. 452; A. Crane, “Notes on the Habits of the Manatees in Captivity in the Brighton Aquarium,” _Proc. Zool. Soc. Lond._ 1881, p. 456. _Extinct Sirenia_:—Gervais, _Journal de Zoologie_, tom. i. p. 332, 1872. R. Lydekker, _Catalogue of Fossil Mammalia in the British Museum_, pt. v. _Order_ CETACEA. This is perhaps the most distinctly circumscribed and natural of all the larger groups into which the class is divided. The external form is fish-like, the body being fusiform, passing anteriorly into the head without any distinct constriction or neck, and posteriorly tapering off gradually towards the extremity of the tail, which is provided with a pair of lateral, pointed expansions of skin supported by dense fibrous tissue, called “flukes,” forming together a horizontally-placed triangular propelling organ, notched in the middle line behind.