10 
MR. W. K. PARKER ON THE STRUCTURE AND 
In this dilemma not only Zoology, but Palaeontology also, fails us utterly, but 
Embryology comes in with every singe and every link. 
I have worked out the early condition of these parts in several kinds of Marsupials, 
and in the young of Ornithorhynchus; but even in the lower Eutheria, the Edentata, 
now to be described, and in the large and varied group of the Insectivora, I have 
been able to trace every step in the transformation of these parts. 
I am now satislied that the incus is the upper element of the first or mandibular 
arch ; both Professor Salensky’s and Professor Fraser’s researches put this, I think, 
beyond doubt ; and my own attempts, for a long time, to make the hyoid theory of 
this part agree with facts, only kept the subject in hopeless confusion. 
The new elements of the ear-chain are, then, the arrested quadrate or incus, and 
the arrested and amputated articular region of the articulo-Meckelian rod, or primary 
lower jaw. The bony part of the “ ramus ’’ is the well-known dentary, with the 
coronoid and splenial bones in a sub-distinct state ; the cartilage for the new articu¬ 
lation of the lower jaw is derived from a large superficial slab—a “lower labial”—the 
like of which is not found again until we get as low down as the Chimaeroids. 
From this is derived the hinder half of the ramus, by transformation of its substance 
into bone ; and from this we get the cartilage, both of the condyle and the glenoid 
cavity, and also of the intervening “ meniscus,” 
Of course the drum cavity is the “ first cleft; ” and the concha auris, with its 
segmented meatus-tube, —the tympanic bone, the tympanic bulla, and the cartilaginous 
lining of the Eustachian tube,—all these are parts of a curiously specialised opercular- 
growth belonging to the hinder edge of the first visceral fold and arch. 
This last assertion has not been made as a stride across the types, from the Mammal 
to the Elasmobranch, but is the result of a very slow, step by step process, made 
during many years, “ along all the lines ” of Vertebrate morphology. 
Vertical section of the slcull o/Tatusia hybrida. — First Stage {embryo, If inch long). 
The chondrocranium is now at its highest development; after this it will begin to 
decline, for osseous centres are already developing in it; these are the basioccipital, 
exoccipital, supraoccipital, basisphenoid, and alisphenoid (Plate 2, fig. 1, b.o., e.o., 
s.o., b.s.). 
The notochord (nc.) is still to be seen in the basioccipital region, it is hooked 
downwards in front. The basioccipital bone (b.o.) occupies the middle half of the 
cartilage between the foramen magnum and the posterior clinoid eievation ; this 
latter is a moderately high ridge running crosswise. 
The cartilage of the base of the skull becomes thicker in front of that ridge, and 
rises gently until it reaches the beginning of the great olfactory fossae; the basi¬ 
sphenoid (b.s.) already occupies the hinder half of this more elevated tract of cartilage. 
