CHAP. Il] THE MAMMALIA 27 



9. The mammary glands are devoid of teats. 



10. The ova are meroblastic, and are discharged in an early 

 stage of their development, enclosed in a tough shell. 



This Sub-class comprises a single living Order, the Monotremata, includ- 

 ing the Duck-bill or Platypus (Ornithorhynchus), and the Spiny Anteater 

 (Echidna). Many features suggest reptilian relationship and ancestry, while 

 other characters are adaptative. Reptilian resemblances are harder to recog- 

 nize in the Theria, which have acquired new features. 



Clavicle 



Humerus 



Fig. 12. Shoulder girdle of a Prototherian Mammal (Echidna) ; to be compared 

 with Fig. 8. Several of the primitive constituent elements of the girdle are still 

 distinct. 



SUB-CLASS II. THERIA. 



1. The vertebral centra possess distinct epiphyses. 



2. The skull bones do not in most cases completely coalesce, 

 most of the sutures remaining distinguishable throughout life. 



3. The coracoid is vestigial [as is also the pro-coracoid] (Fig. 

 10). 



4. The episternum is incorporated in the substance of the 

 presternum (Gotte). 



