30 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jail. 4, 



the separation of which into fourth and third ventricles was indicated 

 only by slight constrictions of the roof and side walls. The large 

 ventricle of the prosencephalon is partially separated into two cham- 

 bers by a median septum, formed by the infolding of its ventral wall ; 

 and the spacious ventricle of each olfactory lobe opens into the dorso- 

 lateral part of each of these chambers. 



The place and mode of origin of the olfactory and of the optic 

 nerves have already been mentioned. 



The third nerve is indicated in the figure ; but I am somewhat 

 doubtful as to the nature of the cord thus marked. 



No fourth or sixth nerve was observed. 



The fifth arises by a single large cylindrical root just below the 

 anterior end of the cornu of the tela vasculosa of the fourth ventricle. 

 The seventh and eighth leave the medulla by a common root just 

 behind this ; and the roots of the ninth and tenth nerves, divided into 

 three bundles, arise from a tract at the sides of the medulla which 

 extends from the last to the hinder limit of the tela vasculosa, and 

 incline obliquely backwards to their exit. 



The brain of Ceratodus is very singular and interesting, inasmuch 

 as it presents resemblances to that of the Marsipobranchii on one 

 side, to that of the Ganoids and Amphibia on another, and to that 

 of the Chimseroids and Plagiostomi on a third. 



As in the brain of the Marsipobranchii, the pineal gland is rela- 

 tively very large, with its pointed dorsal end inclined upwards and 

 Corwards, and the roof of the fourth ventricle is almost entirely 

 formed by the tela vasculosa ; but, as in the Ganoidei and Am- 

 phibia, the cerebellum is larger than in the Lampreys. In Cera- 

 todus it is similar to, though proportionally less than, that of Lepi- 

 dosteus, and still more like that of Polypterits. In the proportions 

 of the thalaniencephalon the brain of Ceratodus resembles that of 

 the Sturgeon and that of the Ray ; while in the representation of 

 the cerebral hemispheres, or prosencephalon, by a large imperfectly 

 divided lobus communis, from the dorso-lateral regions of which the 

 olfactory lobes take their rise, the brain of Ceratodus presents a feature 

 hitherto known, so far as I am aware, only in the Plagiostomi*. 

 Thus, in its cerebral characters, Ceratodus occupies a central place in 

 the class Pisces. 



The development of the cerebral hemispheres in Plagiostome fishes 

 differs from the process by which they arise in the higher Vertebrata. 

 In a very early stage, when the first and second visceral clefts of the 

 embryo of Scyllium are provided with only a few short branchial 



* So far as I can judge from the examination of a small but well-preserved 

 specimen of Lepidosiren annectens, for which I am indebted to Mr. Sclater, the 

 brain of tliis fish is similar, in all essential respects, to that of Ceratodus. The 

 figure of the brain of Lepidosiren given by Prof. Owen in his ' Anatomy of Ver- 

 tebrates ' is susceptible of interpretation in this sense. Hyrtl's description and 

 figure of the brain of Lepidosiren paradoxa (Abhandlungen der koniglichen 

 bohmischen Gesellschaft, Bd. iii. 1845), on the other hand, leave me in doubt 

 whether, apart from its curious asymmeti-y, the brain of this fisli does or does 

 not present important differences from that of Ceratodus wnAihiiX oi L^ido- 

 siren annectens. 



