Zoological Society. 303 



In the summer of 1829 it occurred to Mr. Ogilby to observe that 

 two living individuals of Mycetes Seniculus did not use the extre- 

 mities of their anterior limbs for the purpose of holding objects be- 

 tween the fingers and thumb, as is common among the Quadrumana ; 

 and he ascertained also, on closer examination, that the thumb, as 

 it has generally been considered, was not in these animals opposable 

 to the other fingers, but originated in the same line with them. 

 Struck with the apparent singularity of the fact, he was induced to 

 pay particular attention to all the other animals, referred by zoolo- 

 gists to the Quadrumanous family, to which he had access ; and the 

 continued observation of more than six years has assured him that 

 the non-opposable character of the inner finger of the anterior ex- 

 tremities, which he first observed in the specimens referred to, is 

 not confined to the genus Mycetes, but extends throughout the 

 whole of the genera of the South American Monkeys, individuals of 

 all of which have now been seen by him in the living state. In 

 none of them, consequently, does a true thumb exist on the ante- 

 rior limbs : and as a further consequence it follows, that the whole 

 of them have hitherto been incorrectly referred to the Quadrumana 

 by zoologists generally. There is a solitary exception among de- 

 scriptive writers from this mode of viewing the subject, D'Azara 

 (as Mr. Ogilby has very recently become aware) Jaaving spoken of 

 the anterior extremities of some of the species observed by him as 

 having five fingers originating on the same line with each other : 

 but the statements of that original observer appear, in this respect, 

 either to have been unnoticed by other authors or to have been 

 passed by as undeserving of attention, so entirely were they at va- 

 riance with the preconceived notions of all. 



Of the eight natural genera which include all the knovm Monkeys 

 of the Western Hemisphere, one, Ateles, is entirely destitute of a 

 thumb, or has that member existing only in a rudimentary form be- 

 neath the skin. In five others, Mycetes, Layothrix, Aotus, Pithecia, 

 and Hapale, the anterior thumbs (using the ordinary expression for 

 them) are placed absolutely on the same line with the other fingers, 

 are of the same form with them, act invariably in the same direc- 

 tion, and are totally incapable of being opposed to them. In the two 

 remaining genera, Cebus and Callithrix, the extremities of the an- 

 terior limbs have a greater external resemblance to the hands of Man 

 and of the Monkeys of the Old "World : the internal finger is placed 

 further back than the general line of the other fingers, and has, on 

 that account, when superficially noticed, the semblance of being 

 opposed to them ; but, as has been correctly observed by D'Azara 

 with reference to Ceb. capucinus, it is less separated than in Man : 

 it is, besides, of precisely the same slender form with the rest, is 

 weaker than them, absolutely without power of opposition to them, 

 and habitually acts in the same direction with they. The impres- 

 sion derived from contemplating the liands of the Old World Mon- 

 keys might induce the belief that the extremities of the Cebi are si- 

 milarly constituted : but if the knowledge that in Mycetes, Pithecia, 

 &c., there are no opposable thumbs, lead to a close observation of 



