CHAPTER IV. 



ON THE ARRANGEMENT OF THE CLASSES INTO LARGER 

 GROUPS. 



HAVING now arrived at the end of the list of classes, and 

 obtained a comprehension of the structural features common to, 

 and characteristic of, each class, it will be proper next to discuss 

 the relations of these classes one to another, and to inquire how 

 far they present such common characters as will enable us to 

 group them into larger divisions. 



And, to commence with the highest classes, it is clear that 

 the Mammalia, Aves, and Reptilia are united together by certain 

 very striking features of their development. All possess an 

 amnion and an allantois, and are devoid, throughout life, of any 

 apparatus for breathing the air which is dissolved in water. 

 Thus they have been termed ABRANCHIATE VERTEBRATA, in 

 contradistinction to Pisces and Amphibia, which possess no 

 amnion, nor allantois (or at most a rudimentary one), and, 

 being always provided at a certain period, if not throughout 

 life, with branchiae, have been called BRANCHIATE VEKTEBRATA. 



The Abranchiate, however, form a far less homogenous as- 

 semblage than the branchiate Vertebrata Mammals being so 

 strongly separated from Heptiles and Birds that I am disposed 

 to regard them as constituting one of three primary divisions, 

 or provinces, of the Vertebrata. The structure of the occipital 

 condyles, the structure and mode of articulation of the niandi- 

 bular rami, the presence of mammary glands, and the noii- 

 nucleated red blood-corpuscles appear to separate Mammals as 



