a new Fossil Animal . 
577 
at right angles to the lower part of the chest; in the ornithorynchus, 
however, the form of this bone entirely agrees with that of 
Ichthyosaurus. 
The tcad has a similar apparatus, but consisting of three distinct 
pieces ; the inferior process and either arm of the arch forming 
a separate bone. 
This bone alone we consider as belonging properly to the 
sternum ; we shall therefore call it the sternal arch. 
The inner side of the scapulas is attached to the arms of this 
arch ; in this it agrees most nearly with the ornithorynchus, in 
which however the extremities of the arch do not proceed the 
whole way behind the scapulas, but only afford a small articulating 
face. In the toad, and birds, the extremities of the arch articulate 
between the scapula and clavicle. 
The flattened hatchet-shaped bone, the narrow and excavated 
end of which unites with the scapula to form the glenoidal cavity, 
and which has been considered as analogous to the flat bones of 
the ornithorynchus, and peculiar to these animals, appears very 
obviously, on comparison with the corresponding parts of the 
crocodile, to be a true clavicle, differing only in its proportions, 
being shorter and therefore wider at the flattened end. 
In the crocodile, the toad, and birds, the extremities of the 
scapula and clavicle unite in the same manner to form the glenoidal 
socket ; and all agree closely with the Ichthyosaurus, except, as has 
been observed, in the shorter proportion and broader end of the 
clavicle in the latter. The flattened bones of the ornithorynchus 
should probably be also considered as substitutes for clavicles ; but 
they deviate very widely indeed from the general form, although 
those of the Ichthyosaurus in some respects present an intermediate 
gradation. 
4 d 2 
