310 MR. W. K. PARKER ON THE OSTEOLOGY OF BALAZNICEPS REX. 
is smooth and flat, gently convex externally and slightly concave within; it is at 
least two-thirds of an inch across at its junction with the body of the quadratum. 
The descending anterior margin of this part of the bone is considerably thicker than 
the upper, and it passes downwards and backwards for two-thirds of an inch. The 
bone then suddenly swells out internally, forms first the convex condyle for the distal 
end of the pterygoid, and then sends upwards and backwards a smooth crescentic 
ridge, with its concavity upwards, to the inner side of the posterior division of the great 
upper condyle. Above this ridge, at the centre of the inner face of the quadratum, there 
is a large sulcus, bounded above by a sharp ridge, and containing two or three large 
oval pneumatic foramina. 
The external part of the quadratum is thick, smooth, and convex ; but the outline from 
the anterior upper condyle to the deep round concavity for the nail-like process of the 
quadrato-jugal is concave. The distance between these parts is one inch, and the width 
at the surface of the cavity for the quadrato-jugal is nearly a quarter of an inch. 
Below this cup the bone expands to form the large posterior condyle for articulation 
with the mandible. This condyle passes backwards, inwards, and slightly down- 
wards, being convexo-concavo-convex in its long diameter, and convex across. 
Nearly at right angles to the inner part of the posterior condyle, a double articular 
process passes equally forwards and inwards: its condyloid portions are nearly equal 
and parallel; but the outer lies on the lowest plane, whilst the inner, which is the 
smallest, terminates where the bone is elevated to articulate with the pterygoid. 
Between these large double condyles the base of the tympanic bone is smooth, concave, 
and pierced with several small pneumatic foramina. 
Perhaps no English bird has a deeper ‘ gomphosis’ for the posterior zygomatic arti- 
culation than the Common Heron; but a similar structure exists in the Storks and in 
many fish-eating birds, e. g. the Albatros, whilst this articulation is very shallow in 
the Pelican. In the Parrots it is moderately deep, and in these birds the orbital pro- 
cess is small; whilst the condyloid structures for articulation with the mandible are 
reduced to one large rounded crescentic internal ridge, with a small rudiment of an 
external articular process, just below the deep pit for the quadrato-jugal. Answering 
to this state of the quadratum, the mandible in these birds is deeply scooped to form an 
articular sulcus, which passes forwards, and slightly downwards, and inwards. The 
modifications of this bone in Birds, although gentle, as we pass from family to family, 
are nevertheless almost innumerable ; yet it may be remarked here, that in Struthious 
birds they have none of that elegance which is so conspicuous in typical birds. Pro- 
fessor Huxley in his Croonian Lecture (1858) proves, from the labours of the great 
embryologists, Reichert, Rathke, and Goodsir, and also from his own researches, that 
this ‘os quadratum’ of Birds is the homologue of the ‘incus’ of Mammals, and of that 
lower articular portion of the great hyo-mandibular series of bones in the Fish, which 
has been named ‘jugal’ by Cuvier and ‘ hypo-tympanic’ by Professor Owen. 
