rOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 331 



such situation, since we find it almost uniformly take place in animals inhabiting 

 the water, whether wholly, as this tribe, or occasionally, as the manatee, seal, and 

 white bear: there is however the same structure in the black bear, which I believe 

 never inhabits the water. This perhaps should be considered in another light, 

 as nature keeping up to a certain uniformity in the structure of similar animals; 

 for the black bear in construction of parts is, in every other respect as well as 

 this, like the white bear. 



The capsulae renales are small for the size of the animal, when compared to 

 the human, as indeed they are in most animals. They are fiat, and of an oval 

 figure ; the right lies on the lower and posterior part of the diaphragm somewhat 

 higher than the kidney ; the left is situated lower down, by the side of the aorta, 

 between it and the left kidney. They are composed of 2 substances; the ex- 

 ternal having the direction of its fibres or parts towards the centre; the internal 

 seeming more uniform, and not having so much of the fibrous appearance. 



The blood of animals of this order is, I believe, similar to that of quad- 

 rupeds; but I have an idea that the red globules are in larger proportion. I will 

 not pretend to determine how far this may assist in keeping up the animal heat; 

 but as these animals may be said to live in a very cold climate or atmosphere, and 

 such as readily carries off heat from the body, they may want some help of this 

 kind. It is certain that the quantity of blood in this tribe, and in the seal, is 

 comparatively larger than in the quadruped, and therefore probably amounts to 

 more than that of any other known animal. This tribe differs from fish in 

 having the red blood carried to the extreme parts of the body, as similar to the 

 quadruped. 



The cavity of the thorax is composed of nearly the same parts as in the quad- 

 ruped;' but there appears to be some difference, and the varieties in the different 

 genera are greater. The general cavity is divided into 2, as in the quadruped, 

 by the heart and mediastinum. The heart in this tribe, and in the seal, is pro- 

 bably larger in proportion to their size than in the quadruped, also the blood- 

 vessels, more especially the veins. 



The heart is inclosed in its pericardium, which is attached by a broad surface 

 to the diaphragm, as in the human body. It is composed of 4 cavities,*' 2 auri- 

 cles, and 2 ventricles: it is more fiat than in the quadruped, and adapted to the 

 shape of the chest. The auricles have more fasciculae, and these pass more 

 across the cavity from side to side than in many other animals; besides, being 



* As the circulation is a permanent part of the constitution respecting the class to which the 

 animal belongs, and as the kind of heart corresponds with the circulation, these should be con- 

 sidered in the classing of animals. Thus we have animals whose hearts have only 1 cavity, oUiers 

 with '2, 3, and 4 cavities. — Orig. 



U U 2 



