mm. V. and D_.), while
just behind the dorsal part is the first appearance of the fimbria or
fornix. In addition to the two fissures already named, there is, in
the Echidna, one which in position and mode of formation corresponds
with the Sylvian fissure of higher mammals. Elliot Smith, however,
wisely refuses to homologize it absolutely with that fissure, and
proposes the name of pseudosylvian for it. The pineal body is
rudimentary, and the optic lobes are now, and throughout the Mammalia,
subdivided into four _corpora quadrigemina_.

[Illustration: From _Cat. R.C.S. England_.

FIG. 19.--Ventral and Dorsal Views of the Brain of Ornithorynchus.]

Among the Marsupialia the Tasmanian devil (Sarcophilus) gives a very
good idea of a generalized mammalian brain, and shows a large
development of the parts concerned in the sense of smell. The most
important advance on the monotreme brain is that the calcarine fissure
has now appeared on the posterior part of the mesial surface and
causes a bulging into the ventricle, called the _calcar avis_ or
hippocampus minor, just as the hippocampal fissure causes the
_hippocampus major_ (Gervais, _Nuov. Arch. Mus_. tom. v., 1869;
Ziehen, _Jenaische Denkschr_. Bd. vi., 1897).

[Illustration: From _Cat. R.C.S. England_.

FIG. 20.--Mesial and Lateral Views of the Brain of Ornithorynchus.]

[Illustration: From _Cat. R.C.S. England_.

FIG. 2l.--Mesial and Lateral Views of the Brain of the Tasmanian Devil
(_Sarcophilus_).]

In the Eutheria or mammals above the marsupials, the cerebellum
gradually becomes more complex, owing to the appearance of lateral
lobes between the flocculus and the vermis, as well as the
paraflocculus on the outer side of the flocculus. The corpus callosum
now first appears as a bridge between the neopallia, and its
development leads to the stretching of the hippocampal formation, so
that in the higher mammals the hippocampus is only found in the lower
and back part of the ventricle, while the rudiments of the dorsal part
remain as the _striae longitudinals_ on the corpus callosum. The
dorsal part of the original anterior commissure becomes the fornix,
and the paraterminal area is modified to form the septum lucidum. The
first appearance of the fissure of Rolando is probably in some of the
Carnivora, in which, as the _sulcus crucialis_, it forms the posterior
boundary of the "ursine lozen