CHORD AT A. 



CHAPTER XXVII. 



THE MAMMALIA. 



Sub-Class I. — Prototheria. 



The Prototheria have only one Hving order, though there 

 are reasons for beUeving that certain extinct forms of 

 mammals may belong to this sub-class. , They constitute 

 the order Allotheria, whilst the living types comprise the 

 order Monotremata. 



Their great importance consists in the fact that they are 

 the lowest types of mammals and in many respects they 



form a transition in 

 structure to the rep- 

 tiles. Like most lowly 

 and primitive forms, 

 they also have a num- 

 ber of very specialised 

 features superposed up- 

 on their generalised 

 organisation. 



We have already 

 seen that the division 

 into sub - classes is 

 based upon the mode 

 of reproduction and 

 on the comparative 

 structure of the repro- 

 ductive organs. The 

 features of the sub- 

 class are therefore as 

 follows : — 



I. Reproduction. 

 —It was not till 1884 

 that the egg-laying propensities of these mammals were 

 definitely discovered. The eggs are much larger than 



Fig. 332. — Diagram of the 



FcETAL Membranes of Echidna as 



SEEN IN Cross-section. 



Serosa. 

 / Allantois. 



Amnion. 



The yolk-sac is on the embryo's left and 

 the allantois on the right. 



