PLATE II. 



Common Pigeon {Columha Livia), 



Dissected so as to show, firstly, some of the main points of agreement and difference 

 between Aves, Reptilia, and Mammalia respectively ; and, secondly, the arrange- 

 ment of the principal muscles of flight. 



In the possession of a single aortic trunk, as seen in the figure 

 a little internally to the letter 2h and above the letter q, birds re- 

 semble Mammalsj as they do also in the physiological peculiarity 

 of being warm blooded^ or ' homoeothermal/ A few Mammals 

 resemble all Birds in being testicondous, see h in figure^, and in 

 the possession of two coeca^ see/" in figure, as also in the possession 

 of a coracoid prolonged down to the sternum, see place of origin of 

 muscle 11 in fig'ure. But all Mammals differ from all Birds in that 

 their single aorta crosses their left and not, as shown at g in this 

 figm-e of a Bird, the right bronchus ; in the absence of any indent- 

 ations of the lungs' surface to correspond with the ribs; and in 

 the absence of any external conformation of the kidney in relation 

 to the i:)elvic bones. In all Mammals there is a sbms urogenitaliSf 

 in none do the genital and urinary ducts open separately, as shown 

 here at k and g, into a cloaca common to genital, urinary, and 

 faecal products. These points are as constant as the possession by 

 Mammalia of a hairy integument and of non-nucleated coloured 

 blood corpuscles. The relation of the pancreas to the duodenum, 

 as seen at e, is a minor point, but probably equally distinctive of 

 Birds in opposition both to Mammals and lleptiles. 



In being testicondous ; in the absence of dilierentiation of the 

 structures of the kidney into cortical and medullary portions, and 

 in the supply of blood-vessels to the gland ; and in the absence of 



