77O STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



In Elasmobranchii the cerebrum is an unpaired 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 Lcpidosteus 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 Lepidostcus ; 

 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 Lcpidosteus 

 is hardly less bulky than that of most Teleostei. 



The presence of tori semicirculares on the floor of the mid- 

 brain of Lcpidosteus 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 Lcpi- 

 dosteus as an undoubted Teleostean character. 



On the whole, the brain of Lepidostcus, though preserving its 

 true Ganoid characters, approaches more closely to the brain 

 of the Teleostei than that of any other Ganoid, including even 

 A mia. 



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 Lcpidosteus, probably reappears again, 

 as already indicated, in the higher Amphibia. The presence of 

 the peculiar vesicle attached to the roof of the thalamencephalon 



