In first demonstrating this relationship, it was shown in how many particulars the _Camelidæ_, without losing the essential characters of Ruminantia, manifested a tendency to the Pachydermatous type; and the evidence which the lost genera, _Macrauchenia_ and _Anoplotherium_, bear to a reciprocal transition from the Pachyderms to the Ruminants, through the _Camelidæ_, cannot but be viewed with extreme interest by the Zoologist engaged in the study of the natural affinities of the Animal Kingdom. The Macrauchenia is not less valuable to the Geologist, in reference to the geographical distribution of animal forms. It is well known how unlooked-for and unlikely was the announcement of the existence of an extinct quadruped entombed in the Paris Basin, whose closest affinities were to a genus, (_Tapirus_,) at that time, regarded as exclusively South American. Still greater surprise was excited when a species of the genus _Didelphys_ was discovered to have co-existed in Europe with the _Palæotherium_. Now, on the other hand, we find in South America, besides the Tapir, which is closely allied to the Palæothere,—and the Llama, to which the Anoplothere offers many traces of affinity,—the remains of an extinct Pachyderm, nearly akin to the European genus _Palæotherium_: and, lastly, this Macrauchenia is itself in a remarkable degree a transitional form, and manifests characters which connect it both with the Tapir and the Llama. ADMEASUREMENTS OF THE BONES OF THE MACRAUCHENIA.