OF THE ORNITHORYNCHUS PARADOXUS. 165 
explain sufficiently its functions ; for, by this, the humeral 
part of either extremities is enabled to approach the other ; 
and thus the whole of the anterior extremities can approxi- 
mate much more than if the articulation of the lower pro- 
cess of the scapula had been attached to the sternum with 
the same degree of fixity as the upper or acromial process. 
In addition to the extensive rotatory motion which the dis- 
tance of the acromion from the joint enables the humerus, 
and consequently the extremity, to perform, another kind 
of motion is performed, by means of a moveable articula- 
tion, found only in reptiles and fishes ; I mean the sliding 
semicircular motion performed by the squamous part of the 
scapula upon the flattened portion of the clavicle *. 
Some exceedingly ingenious attempts have been made 
to reduce the whole assemblage of bones composing the 
shoulder, clavicle, and sternum of the ornithorynchus to 
their analogous bones in birds and reptiles, but these at- 
tempts have not as yet been very successful, and the rea- 
son of this appears to be sufficiently obvious for the bones 
of the sternum in reptiles are in general so anomalous as to 
defy classification, founded on analogy with other vertebral 
animals ; whilst the analogies endeavoured to be established 
between the same parts in birds, and the mammalia, are by 
no means generally agreed to. In the ornithorynchus, we 
find, as it were, a compound of the three classes Mammalia, 
Aves and Reptilia. The dorsal part of the scapula re- 
sembles the same part in birds. The acromial process ap- 
proaches that of the Mammalia ; the glenoid cavity of the 
joint is placed at the union of the dorsal part of the sca- 
pula and the process extending to the sternum, which pro- 
* In Plate V,, will be found an accurate sketch of the assemblage of 
bones composing the shoulder and sternum of the Ornithorynchus, accom.- 
panied by one of the fossil animal alluded to in the text. 
