The most salient characteristic of this group of mammals in the estimation of their position in the vertebrate series is not so much the fact that they are oviparous as that the eggs are large-yolked, and develop therefore, so far as regards their early stages, after the fashion of the egg of a reptile. The laying {107} of eggs, or at least ovoviviparity, would follow from the structure of the egg, since the abundance of yolk would do away with the necessity for a placenta. That the eggs had this Saurian characteristic was first definitely made known by Professor Poulton[53] for _Ornithorhynchus_, and his results were confirmed later for _Echidna_.[54] The structure of the eggs has, however, already been dealt with on p. 72. The fact that these animals lay eggs appears to have been known for a very long time, though rediscovered so lately as 1884 by Mr. Caldwell.[55] In connexion with the structure of the ova, the ovaries themselves and the oviducts are built upon the Sauropsidan plan. In the male the testes retain the primitive abdominal position. The fact that the urinary and genital products escape by means of their ducts into a chamber which also receives the end of the alimentary tract is not a distinctive feature of this group, inasmuch as it is seen in the Marsupials, and also in certain low Eutheria, such as the Beaver and other Rodents, and a few Insectivores. As to external features, the Monotremata show certain archaic characters. The unspecialised arrangement of the mammary glands has already been described. These animals are plantigrade, if the term may be used also to describe the aquatic _Ornithorhynchus_. The ears are absolutely destitute of a conch. The remarkable spur upon the hind-legs furnished with a gland, which is more marked in the male, and indeed disappears in the female of _Ornithorhynchus_, is a structure which argues the specialised condition of these two modern representatives of what must have been a large order in the past. {108} [Illustration] FIG. 51.--Ventral view of skull of _Echidna aculeata_, and right half of mandible, _ang_, Angle of mandible; _aud.oss_, auditory ossicles; _cond_, condyle of mandible; _cor_, coronoid process; _max_, maxilla; _oc.cond_, occipital condyle; _pal_, palatine; _p.max_, premaxilla; _pt_, pterygoid; _sq_, squamosal; _ty_, tympanic ring. (After Parker and Haswell.)