It appears to me that, just as among the existing _Carnivora_, the walruses and the eared seals are intercalary forms between the fissipede Carnivora and the ordinary seals, so the Zeuglodonts are intercalary between the _Carnivora_, as a whole, and the _Cetacea_. Whether the Zeuglodonts are also linear types in their relation to these two groups cannot be ascertained, until we have more definite knowledge than we possess at present, respecting the relations in time of the _Carnivora_ and _Cetacea_. Thus far we have been concerned with the intercalary types which occupy the intervals between Families or Orders of the same class; but the investigations which have been carried on by Professor Gegenbaur, Professor Cope, and myself into the structure and relations of the extinct reptilian forms of the _Ornithoscelida_ (or _Dinosauria_ and _Compsognatha_) have brought to light the existence of intercalary forms between what have hitherto been always regarded as very distinct classes of the vertebrate sub-kingdom, namely _Reptilia_ and _Aves_. Whatever inferences may, or may not, be drawn from the fact, it is now an established truth that, in many of these _Ornithoscelida_, the hind limbs and the pelvis are much more similar to those of Birds than they are to those of Reptiles, and that these Bird-reptiles, or Reptile-birds, were more or less completely bipedal. When I addressed you in 1862, I should have been bold indeed had I suggested that palaeontology would before long show us the possibility of a direct transition from the type of the lizard to that of the ostrich. At the present moment, we have, in the _Ornithoscelida_, the intercalary type, which proves that transition to be something more than a possibility; but it is very doubtful whether any of the genera of _Ornithoscelida_ with which we are at present acquainted are the actual linear types by which the transition from the lizard to the bird was effected. These, very probably, are still hidden from us in the older formations.