NATURAL SCIENCES OF PHILADELPHIA. 95 



keys (Platyrrhini and Arctoppithecini) or those of the Old World 

 (Catarrhini). 



Rete Alirahile in Bradypus Didacfylus. Of the man}'' peculi- 

 arities in the organizations of the sloths, one of the most interest- 

 ing is the breaking up of the arteries into rete mirabile. This is 

 well seen in the upper extremity of a two-toed sloth {Bradypus 

 Didacfylus) which recently died at the Zoological Garden, Phila- 

 delphia. While the axillary arterj^ is seen to continue its course 

 as the brachial, diminished, however, in its calibre, it gives off 

 numerous branches which divide and subdivide. The main artery 

 with the surrounding plexus and the median nerve passes through 

 the internal condyle of the humerus, Plate 13, Fig. 3. In this 

 latter respect it differs from the three-toed sloth, as may be seen 

 by comparison with the beautiful plates of Prof. Hyrtl. This in- 

 teresting disposition of the bloodvessels is also seen in the femoral 

 arteries of these animals. Yarious explanations have been offered 

 for the rete mirabile of arteries. Thus in the Cetacea the dividins: 

 and subdividing of the arteries appear to serve as reservoirs of 

 arterialized blood, enabling such animals as the porpoise, etc., to 

 remain for a long time under water. In the sloths and slow lemurs 

 this disposition of the bloodvessels seems to be in relation with 

 the slowness of the circulation, fluids travelling less rapidly 

 through a number of small vessels than one large one. 



On report of the Committee to which it was referred, the fol- 

 lowing paper was ordered to be published : 



