272 MR. W. K. PARKER ON THE STRUCTURE AND 
(1.) The limited, suberect, and flattish cribriform plates, and the small frontals are 
the necessary correlates of a small brain-cavity and brain. The occipital plane corre- 
sponds with those parts, being suberect: it forms but little more than a right angle 
with the general basicranial axis. 
(7.) In the Marsupials, as well as in the Monotremes, we see the “ ossicula audits ” 
i making, so to speak. In all the three subdivisions of this class—Monotremes or 
Prototheria, Marsupials or Metatheria, and Placentals or Eutheria—the lower jaw is 
broken up, the larger front part becoming the persistent mandible, and the shorter 
hind part the malleus, whilst the starved and modified quadrate becomes the cneus. 
For a long while in the growing Marsupial the malleus is manifestly a compound 
bone; it is an “‘articulare” with an internal and a posterior angular process, as in 
the Fowl. On it the “angulare” can be seen, and sometimes, as in the half-grown 
Koala (Phascolarctos), a “supra-angular” too. 
The working mandible attached to a new pier on the jugal and squamosal is com- 
posed of a sort of morphological mixture of a large inferior labial cartilage, a dentary 
bone, with coronoid and splenial regions, and the greater part of MrcKeEn’s cartilage 
—the true primary ramus. 
(j.) The topmost segment of the next arch (pharyngohyal) is often a “columella,” 
and not a stapes. In the early young and embryo of the Marsupials it is V-shaped, 
its greater front fork enlarging above and forming the inverted base of the columella 
or stapes, and the lesser hind fork becoming, after a time, detached and then ossified, 
and forming the interhyal. 
In Fishes, the uppermost element of a branchial arch (and the hyoid is a branchial 
arch) often forks; in the Sturgeon these become two separate pieces, as in this 
particular case of the embryo Margupial. There is not much to remark upon in the 
rest of the hyoid arch, the functional suspensory part. 
For comparison with the Insectivora the existing Marsupials do not yield me all 
the archaic characters I want. 
For the existing low Eutheria are, of course, the descendants of Metatheria that 
were much more generalized and archaic than any now existing ; these latter, during 
the whole Tertiary period, must have undergone, on their own low platform, many 
adaptive changes that would make them look very strange beside the Marsupials of 
the Secondary epoch, if these latter could be restored for comparison. 
The best type of Insectivore for general comparison is the Hedgehog (Hrinaceus 
curopeus), as it shows the least suppression of parts, and the best development of 
that which is diagnostic, so to speak, cf the Order. 
In it the greater investing bones of the skull are similar to those of the Marsupial, 
but the nasals and squamosals are smaller, and the frontals are larger. In the hard 
palate there is a considerable relapse, as in Marsupials, certain tracts of bone being 
absorbed, but it has no mesopterygoids, and only five vomers; yet the antero-lateral 
pairs are well developed. 
