72 INTRODUCTION TO CLASSIFICATION. 



continuous horny cylinder is formed, which constitutes the 

 " quill." 



XXVIII. THE MAMMALIA. 



All Mammals possess an amnion of an essentially similar 

 character to that of Birds and Reptiles, and all have an allan- 

 tois. But the latter either ceases to exist after a very early 

 period of foetal life, or else it is " placentiferous," and serves as 

 the means of intercommunication between the parent and the 

 offspring. Of the nature and characters of the "placenta" 

 developed in the majority of the Mammalia I shall speak more 

 particularly by and by. For the present, I pass it over as a 

 structure not universally characteristic of the class. 



Fig. '67. The occipital condyles of a Dog's skull viewed from behind. Signification of 

 the letters as in Fig. 35. 



The visceral arches are, throughout life, as completely 

 devoid of branchial appendages in Mammals, as in Birds and 

 Reptiles. In the skull, the basi-occipital is well ossified, and, 

 with the ex-occipitals, enters into the formation of the cranio- 

 spinal articulation ; the occipital condyle thus formed, however, 

 is not single, as in Reptiles and Birds, but double, and the atlas 

 has corresponding articular facets. 



Each ramus of the lower jaw is composed of only a single 

 piece, and this articulates directly with the squaniosal bone of 

 the skull, and not with the representative of the quadrate 



