THE MAMMALIA. 7] 



character to that of Birds and Keptiles, and all have an allantois. 

 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 uni- 

 versally characteristic of the class. 



The visceral arches are, throughout life, as completely devoid 

 of branchial appendages in Mammals, as in Birds and Keptiles. 

 In the skull, the basi-occipital is well ossified, and, with the ex- 

 occipitals, enters into the formation of the cranio-spinal articula- 

 tion ; the occipital condyle thus formed, however, is not single, 

 as in Reptiles and Birds, but double, and the atlas has corre- 

 sponding articular facets. 



J-.0 



s 





Fig. 35. — The occipital condyles of a Dog's skull viewed from behind. — Signification of 



the letters as in Fig. 33. 



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

 piece, and this articulates directly with the squamosal bone 

 of the skull, and not with the representative of the quadrate 

 bone. 



The greater and lesser circulations of Mammals are as com- 

 pletely distinct as in Birds, and there is but a single aortic arch, 

 the left. The majority of the blood-corpuscles are red, free 

 nuclei, and these are always discoidal, and usually circular in 

 form. The blood is hot. There is a complete diaphragm, and 

 none of the bronchi end in air-sacs. 



