_Myrmecophaga jubata_; all one-third nat. size. XXV. Fig. 1 and 2. Humerus, radius and ulna of _Scelidotherium_. Fig. 3. Head of humerus. Fig. 4. Head of radius. Fig. 5, 6, and 7. Femur of _Scelidotherium_. XXVI. Fig. 1, 3, and 5. Astragalus of _Megatherium_. Fig. 2, 4, and 6. Astragalus of _Scelidotherium_. XXVII. Fig. 1 and 2. Patella and head of tibia of _Scelidotherium_. Fig. 3, 4, and 5. Ungueal phalanx of _Scelidotherium_. XXVIII. Fig. 1. Astragalus of _Megatherium_. Fig. 2. Astragalus of _Scelidotherium_. Fig. 3–6. Astragalus of _Mylodon_ or _Megalonyx_. XXIX. Lower jaw of _Megalonyx Jeffersonii_. XXX. Fragment of the Cranium of _Megatherium Cuvieri_. XXXI. Section of the upper jaw and teeth of the _Megatherium Cuvieri_. XXXII. Fig. 1. Magnified view of structure of molar of _Megatherium_. Fig. 2–5. Ungueal phalanx and portions of tesselated armour of _Hoplophorus euphractus_. Fig. 6–11. Jaws and teeth of _Ctenomys priscus_. Fig. 12. Bones of right hind-foot of a Rodent. Fig. 13 and 14. Fossil tooth of a Horse. PREFACE. His Majesty’s ship, Beagle, under the command of Captain FitzRoy, was commissioned in July, 1831, for the purpose of surveying the southern parts of America, and afterwards of circumnavigating the world. In consequence of Captain FitzRoy having expressed a desire that some scientific person should be on board, and having offered to give up part of his own accommodations, I volunteered my services; and through the kindness of the hydrographer, Captain Beaufort, my appointment received the sanction of the Admiralty. I must here, as on all other occasions, take the opportunity of publicly acknowledging with gratitude, the obligation under which I lie to Captain FitzRoy, and to all the Officers on board the Beagle, for their constant assistance in my scientific pursuits, and for their uniform kindness to me throughout the voyage. On my return (October, 1836) to England, I found myself in possession of a large collection of specimens in various branches of natural history; but from the great expense necessary to secure their publication, I was without the means of rendering them generally serviceable. The Presidents of the Linnean, Zoological, and Geological Societies, having given me their opinion respecting the utility to be derived from publishing these materials, I addressed a letter to the Right Honourable the Chancellor of the Exchequer (T. Spring Rice, Esq.) informing him of the circumstances under which I hoped that I might venture to solicit the aid of Government. In reply, I received a communication (as below) announcing to me that the Lords of the Treasury, from their readiness to promote Science, were willing, under certain conditions, to give me the most liberal assistance.