560 PROFESSOR OWEN ON THE GENUS DINORNIS. 
The tympanic’ (Pl. XL. fig. 1, 28, Pl. XLI. figs. 2, 3, 4) has its mastoid end expanded 
and divided into two convex condyles (figs. 2 & 4, #), corresponding in form and relative 
size to the cavities for their reception above mentioned. The inner, larger, oblong 
condyle is more convex lengthwise than transversely; the outer, smaller condyle is 
subhemispheric; they are connected by a narrow tract, forming the upper part of a 
pneumatic foramen which divides the condyles from each other posteriorly. The stem 
of the condyles, subcompressed, from 3 lines to 4 lines in length, quickly expands as it 
descends, sending off anteriorly a broad, thin, compressed, triangular, obtusely pointed 
orbital plate (ib. ¢), and downward and backward outswelling in every direction, but 
chiefly antero-posteriorly, to form an unusually large lower end (¢), the anterior half of 
which affords the single articular surface (Pl. XLI. fig. 3, 7) for the mandible. On the 
outside of the base of the articular stem is the process with the flat articular surface 
(fig. 4, f) for the premastoid ; on the outside of the mandibular articular expansion is 
the articular cavity (ib. 2) for the squamosal. ‘The orbital plate is widely excavated 
internally; and the lower border of the cavity sends off the small hemispherical arti- 
cular tubercle (fig. 2, 7) for the pterygoid. Thus there are not fewer than five articular 
facets on this singularly modified homologue of the element or “ process of the tem- 
1 In regard to the homology indicated by this name, I have no better reason for breaking a silence which an 
accuser might call obstinate, save the following, which I give for what it is worth. Prof. Huxley states (Proc. 
Roy. Soc. Noy. 18, 1858), with the emphasis of italics, and a repetition of negatives, implying sense of the 
insecurity of his ground, “that the tympanic of the mammal does not articulate with the lower jaw, nor with 
the pterygoid, nor with the jugal or quadrato-jugal;” and so trite a statement of a commonly known fact 
would have remained unnoticed by me if it had not been quoted, in a former yolume of the ‘ Transactions of 
the Zoological Society,’ with commendation, as if it were a novel contribution to the elements for determining 
the homology of that bone in other vertebrates. Mr. Parker puts this statement, which he rightly characterizes 
as “ yery true,” in the van of his arguments for opining “that the quadratum of birds is the homologue of the 
mammalian incus” (Trans. Zool. Soc. iv. p, 316). 
But the “‘incus” “ of the mammal does not articulate with the lower jaw, nor with the pterygoid, nor with 
the jugal, or quadrato-jugal.” What is more, also, and what Mr. Parker’s guide was careful to be silent upon, 
is this, viz. that the incus of the mammal does not articulate with the mastoid, or with the squamosal, nor does 
it support the membrana tympani or any part thereof. No doubt the tympanic of the mammal is reduced in 
divers degrees in that class; but it always retains those relations with the mastoid and squamosal, and per- 
forms that function in reference to the ear organ, which characterize it under all its subordinate and accessory 
developments in Birds, Chelonians, Crocodiles, and other air-breathing vertebrates. The averment that “ Pro- 
fessor Owen, once wrong, goes far astray’’ (Parker, ibid. p. 304), does not affect the facts nor the legitimate 
deductions from them which guide to a recognition of the homology of the tympanic, the mastoid, and the 
squamosal—at least by one bent “on ascertaining, instead of pleasantly supposing, the true nature of an ana- 
tomical element” (Parker, ib. p. 271). The case is this: the negative argument tells as strongly against the 
incus or other member in the chain of ossicles or gristles connecting the membrana fenestre vestibuli with the 
membrana tympani, as it does against the tympanic, whilst the positive evidence is exclusively in favour of the 
tympanic. 
One use of homology or ‘‘ namesakeism ” is to rid anatomy of different names for the same thing. Why do 
not those who believe the ‘‘ quadratum ” to be the “‘incus” or the “ malleus” call it one or the other in the 
bird and hematoerya, or else call the mammalian otosteal of their choice ‘ quadratum” ? 
