54 GFNBS/S OF MAN. 



agree with the birds in having the anterior extremities of the 

 clavicles united to the sternum, forming a sort of merrythought. 

 A still more fundamental point of resemblance to the birds, and 

 that from which the sub-class takes its name, is the possession of 

 a common cloaca, the urino-genital duct opening within the body. 

 The monotremes, however, agree with the mammals in all the 

 characteristic attributes above enumerated, such as double occipital 

 condyle, complete diaphragm, etc., while the cloaca, the merry- 

 thought, and other apparently avian characters, may have been 

 inherited as well from the amphibians as from the birds. The 

 beak, however, can only be accounted for as having developed 

 independently from adaptation to conditions of existence similar to 

 those which evolved the toothless jaws of turtles, from which it is 

 believed the beak of birds has been derived. The beak of the 

 Echidna differs from that of the Ornithorhynchus , and exhibits an 

 approach towards the snout of the ant-eaters. The beaks of 

 monotremes and of birds must therefore be regarded as simply 

 analogous, and not as homologous organs. 



The Promammalia no doubt differed in many respects from the 

 Monotremata, and Haeckel is inclined to believe that they pos- 

 sessed regular teeth, which the latter lost through adaptive modifi- 

 cation. At least, the earliest fossil remains that paleontologists 

 have been able to refer with certainty to the mammals, and which 

 occur in the triassic formation, consist of teeth only. From a few 

 small molar teeth found in Germany, and also in England, Micro- 

 lestes antiquns has been constructed; and from similar dental 

 remains found in this country, Dromatherium sylvestre has been 

 described. 



Although the Monotremata differ from the Monocondylia (rep- 

 tiles and birds) in so many important respects in which they agree 

 with the higher mammals, they, nevertheless, also present many 

 points of difference with these latter. In addition to those already 

 mentioned (cloaca, united claviculae, etc.), the absence in these 

 animals of any teats upon the mammary glands is very peculiar 

 and anomalous. In consequence of this omission, the only way 

 in which the young are able to obtain their nourishment, is by a 

 process of licking against the porous breast of the mother ; and 

 Haeckel, therefore, proposes as a synonym for the ordinary name 

 of the sub-class, that of Amasta, — or mammals without teats. 



