270 
ME. W. K. PARKER ON THE STRUCTURE AND 
For in these, whether they be Eastern or Western types, the uniformity on the 
whole is very remarkable—as remarkable as the diversity seen in the Insectivora. 
The problem put to the Morphologist, however, is to explain why the characters 
that distinguish a Marsupial from a high or Eutherian Mammal are, for the most 
part, characters which the former possess in common with the Sauropsida: the residuum 
of proper, unique Metatherian characters, neither to be found in the higher Mammals 
on one hand, or in the Sauropsida on the other, is but small. 
Another crucial difficulty is this, namely, the Sauropsida, which of all others help 
us most in the interpretation of the Marsupial skull, are not those to be found in low 
Reptilian, but in the highest Avian, types. 
Of all Birds the Passerinse are the noblest and most marvellously specialized for 
their own peculiar mode of life, having many accomplishments and high intelligence. 
Yet it is from this Order of Birds that I have had most help in this matter, finding 
in their skull special structures which closely correspond with what is most remarkable 
in that of the Marsupials. 
There are several characters in the superficial or investing elements of the skull of 
Marsupials that are unlike what we find in the highest forms of placental Mammalia, 
but which linger in the lower :— 
{a.) The frontals are very small in proportion to the parietals, and the squamosals 
are relatively, especially in the young, inordinately large, as large as the frontals. 
( b .) The lachrymals are not only large and have generally a facial plate, but they 
have, as a rule, two canals. 
(c.) The palatine plates of the maxillaries and palatine bones form an extremely 
hollow or dome-like structure, and by the time the creature is full-grown much of 
their substance has been absorbed, so as to leave larger or smaller fenestrae ; thus there 
is an attempt to return to the schizognathous condition of those parts seen in many 
Sauropsida. 
(cl.) The palatines are often formed of several pieces, very irregular patches of bone, 
and their irregular centres are largely absorbed or united with the main parts in the 
adult. 
(<?.) The pterygoids are very small, and their basicranial part limited, on account of 
the constant separate development of a large mesopterygoid. 
(f.) The main vomer is often relatively small; there is, nearly always, a pair of 
antero-lateral vomers, protecting the cartilaginous capsule of Jacobson’s organs, and 
large postero-lateral, and other, or postero-medial vomers ; these are very irregular and 
unsymmetrical in the young Cuscus, especially, in which I find ten vomerine bones. 
(g.) The floor of the tympanic cavity ossifies before the cartilage is ripe, but in two 
subequal centres—the annulus and “ os bullae ”— inside the latter a larger folded 
cartilage protects the Eustachian tube, and outside the former the meatus externus is 
protected by a more or less segmented tube of cartilage, which ends outside in the 
continuous concha auris. 
