72 R. W. SHUFELDT. 



injected into their months by a special muscle of the mammary 

 glands. 



By far the great hulk of existing mammals belong to the 

 Eutheria or Placental Mammals. Throughout this great group 

 the young, or foetus, is invariably nourished during its develop- 

 ment within the womb of the mother, by its attachment through 

 an umbilical cord, with an allantoic placenta. Exceptions to this 

 rule are merely to be regarded in the light of accidents, as in 

 the case of extra-uterine pregnacies, and other departures from 

 the normal course. 



While the Prototheria and MetcUheria have practically come 

 to a standstill as groups of mammals, in so far as further deve- 

 lopment is concerned, the Eutheria, on the other hand, is still 

 evolving with a powerful impulse toward specialization in mor- 

 phology, mentality, and general organization. 



It includes mammals of the widest possible variation in 

 form, as men and mice, bats and whales, elephants and shrews, — 

 yet they are all more or less linked together by anatomical and 

 other bonds, the interpretation of which there can now be no 

 shadow of a doubt. 



Among existing Eutheria there are at least three groups 

 that can at once be differentiated from all the others; these are 

 the Edentata, containing the Armadillos, Sloths, and Anteaters; 

 next the Sirenia, containing such large aquatic herbivorous 

 mammals as the Dugongs, the Manatees, and some fossil forms 

 closely aUied to them, as the Rhytina and others; finally, the 

 Cetacea, a group containing the various species of Whales, Por- 

 poises, and Dolphins. 



With respect to the remaining groups it is clear that they 

 are more nearly related to each other, notwithstanding the great 

 diversity of form to be seen among them. Although endowed 

 with wonderful differences in the matter of their dentition, the 

 dental formula is, nevertheless, the same throughout, — that is 

 to say, all in the permanent dentition, whatever their modifi- 



