Pterodactyles all have prepubic bones, which are only known in Ornithorhynchus and Echidna among mammals, and are absent from the higher mammals and birds.

The Ornithorhynchus, on the other hand, lays from two to four eggs, which in size and general appearance resemble those of the echidna.

The mammary glands of the Ornithorhynchus may, perhaps, be considered, in comparison with the udder of a cow, as in a nascent state.