The scapula resembles that of the cetacean Hyperoodon, but the supra-spinous fossa is larger and more seal-like; as is the humerus, which differs from that of the Cetacea in presenting true articular surfaces for the free jointing of the bones of the fore-arm.  In the apparently complete absence of hinder limbs, and in the characters of the vertebral column, the Zeuglodon lies on the cetacean side of the boundary line; so that upon the whole, the Zeuglodonts, transitional as they are, are conveniently retained in the cetacean order.  And the publication, in 1864, of M. Van Beneden’s memoir on the Miocene and Pliocene Squalodon, furnished much better means than anatomists previously possessed of fitting in another link of the chain which connects the existing Cetacea with Zeuglodon.  The teeth are much more numerous, although the molars exhibit the zeuglodont double fang; the nasal bones are very short, and the upper surface of the rostrum presents the groove, filled up during life by the prolongation of the ethmoidal cartilage, which is so characteristic of the majority of the Cetacea.

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.