CHARACTERS OF MONOTREMA. 235 



bone, have become united into one piece, similar to the well- 

 known fork-bone, or merry-thought, in birds. In all other 

 Mammals the two collar-bones remain separated in front 

 and do not fuse with the breast bone. Moreover, the 

 coracoid bones are much more strongly developed in the 

 Cloacal animals than in the other Mammalia, and are con- 

 nected with the breast bone. 



In many other characteristics also — especially in the 

 formation of their internal genital organs, their auricular 

 labyrinth, and their brain — Beaked animals are more closely 

 allied to the other Vertebrata than to Mammals, so that some 

 naturalists have been inclined to separate them from the 

 latter as a special class. However, like aU other Mammals, 

 they bring forth living young ones, which for a time are 

 nourished with milk from the mother. But whereas in all 

 other Mammals the milk issues through nipples, or teats, 

 from the mammary glands, teats are completely wanting 

 in beaked animals, and the milk comes simply out of a flat, 

 sieve-like, perforated patch of the skin. Hence they may 

 also be called Breastless or Teatless animals (Amasta). 



The curious formation of the beak in the two stiU living 

 Beaked animals, which is connected with the suppression 

 of the teeth, must evidently not be looked upon as an 

 essential feature of the whole sub-class of Cloacal animals, 

 but as an accidental character of adaptation distinguishing 

 the last remnant of the class as much from the extinct main 

 group, as the formation of a similar toothless snout dis- 

 tinguishes many toothless animals (for instance, the ant- 

 eater) from the other placental animals. The unknown, 

 extinct Primary Mammals, or Promammalia — which lived 

 during the Trias period, and of which the two still living 



