MO NOTRE MAT A ^(^y 



but they retain many reptilian features, such as separate ptery- 

 goids postorbitals and pre vomers, large clavicles and an inter- 

 clavich-, and both coracoids and precoracoids (Fig. 390). They have 

 no corpus callosum ; their temperature, although higher than that 

 of their surroundings, is variable ; and they possess a cloaca. 

 Above all, they lay large yolky eggs. After hatching, the young 

 is nourished in a pouch by milk produced in specialised sweat 

 glands. Both genera are specialised, Echidna with its spines and 

 ant-eating habits, and the platypus 

 for aquatic life. Neither has teeth. 



Neither marsupials nor monotremes 

 can be looked on as ancestral 

 mammals, and fossils give us little 

 indication of their connection with 

 the placental stock. A reduction in 

 the lower jaw can be traced in a 

 number of fossil reptiles of the group 

 Synapsida, and from these we may 

 assume the mammals to have come. 

 There are other extinct groups which 

 have only the dentary in the lower 

 jaw, and so are classed as mammals, 

 but we have no means of telling if 

 they had hair and warm blood or 

 how they fed their young, so that 

 we cannot tell when mammals, as we 

 know them to-day, originated. Unless 

 milk, hair and temperature control 

 arose together three times over, the 

 three modern groups must all be descended from the first 

 mammals, but they early diverged. The marsupials show signs 

 of having lost a placenta, and so perhaps they left the 

 eutherian line soon after it was established, but the monotremes 

 have a remoter connection with the other two. If we wish to 

 have a mental picture of a prototypical mammal we must think of 

 a creature which combines the teeth, habits and general build 

 of an insectivore with the egg-laying and other reptilian features 

 of the platypus. Such a creature perhaps lived a hundred 

 million years ago at the end of the Jurassic period. 



Fir;. 390. The shoulder ^irdle 

 and breastbone of a duck- 

 mole. 



cl.. Clavicle ; cor., coracoid ; g.c, glenoid 

 cavity ; i.cL, interclavicle ; p.c., 

 precoracoid ; r., ribs ; 5C., scapula; 

 St., stemebrae. 



