The line of descent of the camels was first indicated in the Proceedings of the Philadelphia Academy, 1875, p. 262; then in the Bulletin No. I, 1874, p. 25, of the U.S. Geological Surveys of the Territories, (Government Report of Explorations of 1873, pp. 498-500); and lastly in U. S. Geographical Surveys of Territories, vol. iv., pp. 341-44. In the above writings, Professor Cope traces the development of the modern camels from the miocene genus _Poëbrotherium_, Leidy, showing the modifications found in _Procamelus_, Leidy, and _Protolabis_, Cope, of the succeeding age. From the last work (Wheeler's Survey, vol. iv., p. 342), we will quote at length: "The evolution of the existing types of Camelidæ is a good illustration of the operation of the laws of acceleration and retardation. In evidence of this we may follow the growth of the foot and dentition of the most specialized, and therefore the terminal genus of the series, the American _Auchenia_ ----. It is well known since the time of Goodsir, that the embryos of ruminants exhibit a series of superior incisor teeth which disappear early. It is probable, but not certain, that in the miocene genus _Poëbrotherium_, as in various contemporary selenodont artiodactyla, the superior incisors persisted. I have, however, discovered that these teeth persisted in the Loup-Fork genus _Protolabis_ during adult life. I have found that one (the second) of these teeth in _Procamelus occidentalis_ persisted, without being protruded from the alveolus, until nearly adult age. "In genera (_e.g._, the bunodont artiodactyla) where the incisors are normally developed, they appear at about the same time with the other teeth, and continue to develop to functional completeness. This development is retarded in _Protolabis_, since they are not so matured as to remain fixed throughout life in their alveoli. "In _Procamelus_, the retardation is still greater, since the first incisor reaches very small dimensions, and is, with its alveolus, early removed; while the second incisor only grows large enough, and for a sufficient time, to occupy a shallow alveolus, without extending beyond it. In the first incisor the process of retardation has reached its necessary termination, _i.e._, atrophy or extinction; while in the existing _Camelidæ_ the second incisor also has disappeared the same way. In ruminants other than _Camelidæ_, the third or external incisor has undergone the same process, while in the _Bovidæ_ the canines also have been retarded in development, down to atrophy." Professor Cope continuing the discussion of the teeth of these types, generalizes: "From these and many analogous cases, the general law may be deduced that _identical modifications of structure, constituting evolution of types, have supervenea on distinct lines of descent?_" This summary of Professor Cope's conclusions has been necessary to show clearly in this series the probable place of the eocene genus.