382 
MESSES. E. M. BALFOUR AND W. N. PARKER ON THE 
Lepidosiren , HyrtiA) ; in either case the hemispheres are traversed for the whole 
length by lateral ventricles which are either completely or nearly completely 
separated from each other. 
In Elasmobranchii the cerebrum is an impaired though bilobed body, but traversed 
by two completely separated lateral ventricles, and without a trace of the peculiar 
membranous roof found in Ganoids. 
Not less interesting than the distinguishing characters of the Ganoid brain are 
those cerebral characters which indicate affinities between Lepidosteus and other groups. 
The most striking of these are, as might have been anticipated, in the direction 
of the Teleostei. 
Although the foremost division of the brain is very dissimilar in the two groups, 
yet the hind-brain in many Ganoids and the mid-brain also in Lepidosteus approaches 
closely to the Teleostean type. The most essential feature of the cerebellum in 
Teleostei is its prolongation forwards into the ventricles of the optic vesicles as the 
valvula cerebelli. We have already seen that there is a homologous part of the 
cerebellum in Lepidosteus; Stannius also describes this part in the Sturgeon, but no 
such part is represented in Muller’s figure of the brain of Polypterus , or described by 
him in the text. 
The cerebellum is in most Ganoids relatively smaller, and this is even the case with 
Amia; but the cerebellum of Lepidosteus is hardly less bulky than that of most 
Teleostei. 
The presence of tori semicirculares on the floor of the mid-brain of Lepidosteus again 
undoubtedly indicates its affinities with the Teleostei, and such processes are stated 
by Stannius to be absent in the Sturgeon, and have not, so far as we are aware, been 
described in other Ganoids. Lastly we may point to the presence of well-developed 
lobi inferiores in the brain of Lepidosteus as an undoubted Teleostean character. 
On the whole, the brain of Lepidosteus, though preserving its true Ganoid characters, 
approaches more closely to the brain of the Teleostei than that of any other Ganoid, 
including even Amia. 
It is not easy to point elsewhere to such marked resemblances of the Ganoid brain, 
as to the brain of the Teleostei. 
The division of the cerebrum into anterior and posterior lobes, which is found in 
Lepidosteus , probably reappears again, as already indicated, in the higher Amphibia. 
The presence of the peculiar vesicle attached to the roof of the thalamencephalon has 
its parallel in the brain of iProtopterus, and as pointing in the same direction a general 
similarity in the appearance of the brain of Polypterus to that of the Dipnoi may be 
mentioned. 
There appears to us to be in no points a close resemblance between the brain of 
Ganoids and that of Elasmobranchii. 
* ‘ Leyidosireu- yaradoxa.’ Prag., 1845. 
