STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 769 



The presence of a largely developed thalamencephajpn at 

 once distinguishes a Ganoid brain from that of a Teleostean 

 Fish, in which the optic thalami are very much reduced ; but 

 Lepidosteus shews 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 some- 

 what everted (cf. Wilder), and in Lepidosteus these edges have in 

 the adult a very great development, and form (vide Plate 38, fig. 

 47 A C, /.) 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 (Protoptcrus, Wiedersheim 1 ) 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 (Ccratodus, Huxley 2 , Lepidosiren, Hyrtl 3 ) ; 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. 



l. Stiidicn, in. Jena, 1880. 

 2 "On Ceratodns Forsleri," Proc. Zool. Soc. 1876. 

 ;| Ltpidosireit paradexa. Prag. 1845. 



