54 OSBORN. [VoL. I. 
diencephalon. In the Salamandride the mesencephalon is 
larger than the diencephalon, and it is of much greater propor- 
tionate size in the Azura, owing to the large optic nerve nuclei. 
The size of the roof of the jmesencephalonms aim 
direct proportion to the functional perfection yar 
the eyes. The cerebellum is invariably small in the Uvodela 
and Protezda, but attains a segmental size and importance in the 
Anura. The metencephalon, or Medulla oblongata, forms 
the fifth, and invariably the largest segment. 
The proportions of these segments are very similar to those 
observed in the Dzpfuoz (see Fulliquet, ’86), but differ widely 
from those in the lower fishes. The great relative size of the 
prosencephalon in the Amphibia is, however, deceptive, since 
the walls are comparatively thin and the ventricles much larger 
than in the two mid-segments. 
Surface characters of the brain, Plate IV. In carefully pre- 
pared specimens many of the more important structures of the 
superficial encephalic walls may be seen upon the surface. 
In the hemispheres of Szvez, Fig. 5, are observed nerve tracts 
from the postero-lateral regions. Between the hemispheres pos- 
teriorly lies the large supraplexus which covers the roof of 
the ventriculus communis, Fig. 8, and in most of the Uvodela 
extends far forwards. The inner surfaces of the hemispheres 
are in close contact with each other in the Uvodela and Proteida, 
but I have observed no actual union of the olfactory lobes such 
as is found in the Azura. The lamina terminalis, 4, Fig. 3, 
extends well forwards, and is sharply defined in the ventral 
aspect. 
The characters of the roof of the Diencephalon are very 
well marked. Just behind the supraplexus are two oval or round 
swellings, which represent the ganglia habenarum with 
distinct median contours, but closely applied to each other in 
forms in which the supracommissura is-well developed, 
such as Menopoma, Amphiuma (Osborn, ’83 and ’84, Fig. 4), 
and Axolotl/, Fig. 1. Arching between them and spreading be- 
yond, into the thalami, is a grayish band which probably repre- 
sents the supracommissura. This commissure underlies 
the pineal process, and is not seen upon the surface in Wecturus, 
Figs. 2 and 8, but is apparently present in Proteus. Immediately 
behind this the whitish plates connected with the ganglia diverge, 
