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 all 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 still 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 
