STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
381 
has called attention to certain characteristic features of the cerebrum which have an 
undoubted systematic value. 
The distinctive characters of the Ganoid brain are, in our opinion, (l) the great 
elongation of the region of the thalamencephalon ; and (2) the unpaired condition of 
the posterior part of the cerebrum, and the presence of so thin a roof to the ventricle 
of this part as to cause it to appear open above. 
The immense length of the region of the thalamencephalon is a feature in the 
Ganoid brain which must at once strike any one who examines figures of the brains 
of Chondrostrei, Polypterus, or Amia. It is less striking in the adult Lepidosteus, 
though here also we have shown that the thalamencephalon is really very greatly 
developed ; but in the larva of Lepidosteus this feature is still better marked, so that 
the brain of the larva may be described as being more characteristically Ganoid than 
that of the adult. 
The presence of a largely developed thalamencephalon at once distinguishes a 
Ganoid brain from that of a Teleostean Fish, in which the optic thalami are very 
much reduced; but Lepidosteus shows its Teleostean affinities by a commencing 
reduction of this part of the brain. 
The large size of the thalamencephalon is also characteristic of the Ganoid brain in 
comparison with the brain of the Dipnoi; but is not however so very much more 
marked in the Ganoids than it is in some Elasmobranchii. 
On the whole, we may consider the retention of a large thalamencephalon as a 
primitive character. 
The second feature which we have given as characteristic of the Ganoid brain is 
essentially that which has been insisted upon by Wilder, though somewhat differently 
expressed by him. 
The simplest condition of the cerebrum is that found in the larva of Lepidosteus , 
where there is an anterior pair of lobes, and an undivided posterior portion with a 
simple prolongation of the third ventricle, and a very thin roof. The dorsal edges of 
the posterior portion, adjoining the thin roof, usually become somewhat everted (cf. 
Wilder), and in Lepidosteus these edges have in the adult a very great develop¬ 
ment, and form (vide Plate 25, fig. 47 A-C, ce.) two prominent lobes, which we have 
spoken of as the posterior cerebral lobes. 
These characters of the cerebrum are perhaps even more distinctive than those of 
the thalamencephalon. 
In Teleostei the cerebrum appears to be completely divided into two hemispheres, 
which are, however, all but solid, the lateral ventricles being only prolonged into their 
bases. In Dipnoi again there is either (Protopterus, Wiedersheim*) a completely 
separated pair of oval hemispheres, not unlike those of the lower Amphibia, or the 
oval hemispheres are not completely separated from each other ( Ceratodus , Huxley, f 
* ‘ Morphol. Studien,’ iii. Jena, 1880. 
t “ On Ceratodus Fonteri Proc. Zool. Soc., 1876. 
