EGGS OF PLATYPUS 107 



of eggs, or at least ovoviviparity, would follow from the struc- 

 ture of the egg, since the ahundance of yolk would do away 

 with the necessity for a placenta. That the eggs had this 

 Saurian characteristic was first definitely made known by Pro- 

 fessor Poulton ^ for Ornifhorhynchus, and his results were con- 

 firmed later for Echidna? 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.^ In connexion with the structure of the ova, the 

 ovaries themselves and the oviducts are built upon the Saurop- 

 sidau plan. In the male the testes retain the primitive ab- 

 dominal position. The fact that the urinary and genital 

 products escape by means of their ducts into a chamber which 

 also receives the end of tlie 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 

 Eodents, and a few Insectivores. As to external features, the 

 ]\Ionotremata 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 tlie past. 

 The skeleton shows numerous ancient characteristics. In 

 the skull there is no demarcation of the orbit from the temporal 

 fossa, a feature widely found in archaic mammals. The tympanic 

 remains as a slender ring, there being no auditory bulla fornled 

 either from this or from any other bone. The malleus and 

 incus are large, and thus reminiscent of the quadrate and 

 articular bone of reptiles. In the lower jaw the absence of 

 a marked coronoid process, and the absence of a firm ossification 

 at the meeting of the two rami, may be a primitive state of 



' iJiinH. Journ. Micr. Sci. xxiv. 1884, p. 124. 

 - Beddard, Proc. Roy. Phys. Soc. Edinh. viii. 1885, p. 354. 



^ See Phil. Trans, clxxviii. 1887, where the literature of the subject is fully 

 cited. 



