768 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



whole length instead of being confined to their proximal portions. 

 If, therefore, our identifications of the larval parts of the brain 

 are correct, there can hardly be a question as to our identifications 

 of the parts in the adult. As concerns these identifications, the 

 comparison of the brain of our two larvae appears conclusive in 

 favour of regarding the anterior lobes as parts of the cerebrum, 

 as distinguished from the olfactory lobes, in that they are clearly 

 derived from the undivided anterior portion of the cerebrum of 

 the younger larva. 



The comparison of the larval brain with that of the adult 

 again appears to us to leave no doubt that the vesicle attached 

 to the roof of the thalamencephalon in the adult is the same 

 structure as the bilobed outgrowth of this roof in the larva ; and 

 since there is in addition a well-developed pineal gland in the 

 larva with the usual relations, there can be no ground for identify- 

 ing the vesicle in the adult with the pineal gland. 



Muller, in his often quoted memoir (No. 13), states that the 

 brains of Ganoids are peculiar and distinct from those both of 

 Teleostei and Elasmobranchii ; but in addition to pointing out 

 that the optic nerves form a chiasma he does not particularly 

 mention the features, to which he alludes in general terms. More 

 recently Wilder (No. 15) has returned to this subject; and 

 though, as we have already had occasion to point out, we cannot 

 accept all his identifications of the parts of the Ganoid brain, yet 

 he 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, (i) the great elongation of the region of the thalamen- 

 cephalon ; 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 Chondrostei, Polyptems, 

 or Ainia. It is less striking in the adult Lepidosteus, though here 

 also we have shewn 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. 



