IV.–On the evolution of the proboscidea

Abstract
The Proboscidea form one of the most isolated groups of the mammalia, for although it is now generally recognised that they belong to the Ungulata, they nevertheless differ widely from all other members of that order. They are further remarkable for the fact that, while in some respects, e. g ., in their large size and peculiar dentition, they are among the most specialised of mammals, in others, as, for instance, in the structure of their feet, they are relatively very primitive. The history of such an isolated group is necessarily of exceptional interest, and the peculiarities of the animals themselves render it the more easy to follow the course of their development, because the danger of confusing them with allied types is to a great extent eliminated; furthermore, in scarcely any other group can the phylogenetic history be traced back through so long a series of forms. Until quite recently the earliest Proboscidea known were from the lowest Miocene beds (Burdigalien) of France, where they are represented by at least two species, viz., Dinotherium cuvieri and Tetrabelodon angustidens. In somewhat later deposits these two genera Dinotherium and Tetrabelodon occur widely distributed in Europe and as far east as India. Tetrabelodon angustidens has also been recorded from the northern part of the Libyan Desert and from Morocco. The group seems to have reached North America during the Miocene, and the earliest recorded species is found in the Upper Miocene beds of Montana. This form has been described by Cope under the name Tetrabelodon brevidens ; it is remarkable for the simplicity of its molars. Dinotherium is not found in any American deposits. The Mastodons did not penetrate into South America till the Pliocene. The absence of Dinotherium from America and the circumstance that the teeth of Tetrabelodon brevidens are at least as simple as those of any European species, suggests the possibility that the Proboscidea may have reached North America not by way of Europe, but along some independent land connection. The absence of the dinotheroid type might also be accounted for by the fact that these animals were very heavily built and probably semi-aquatic, so that their spread into new areas may have been very slow and perhaps was checked by barriers that would not interfere with the Tetrabelodonts.

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