STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 767 



prominences of the floor of the ventricles of the optic_Jo_bes, 

 which we regard as homologous with the tori semicirculares of 

 the brain of the Teleostei. 



The hind-brain is formed of the usual divisions, viz. : cere- 

 bellum and medulla oblongata (Plate 37, figs. 44 and 45, cb.,md^). 

 The former constitutes a bilobed projection of the roof of the 

 hind-brain. Only a small portion of it is during these stages left 

 uncovered by the optic lobes, but the major part extends forwards 

 for a considerable distance under the optic lobes, as shewn in 

 the transverse sections (Plate 37, figs. 46 F and G, cb.) ; and 

 its two lobes, each with a prolongation of its cavity, are con- 

 tinued forwards beyond the opening of the iter into the fourth 

 ventricle. 



It is probable that the anterior horns of the cerebellum are 

 equivalent to the prolongations of the cerebellum into the central 

 cavity of the optic lobes of Teleostei, which are continuous with 

 the so-called fornix of Gottsche. 



III. Comparison of the larval and adult brain of Lepidosteus, 

 togetJier with some observations on tJie systematic value of the 

 characters of the Ganoid brain. 



The brain of the older of the two larvae, which we have 

 described, sufficiently resembles in most of its features that of 

 the adult to render material assistance in the interpretation of 

 certain of the parts of the latter. It will be remembered that in 

 the adult brain the parts usually held to be olfactory lobes were 

 described as the anterior cerebral lobes. The grounds for this 

 will be apparent by a comparison of the cerebrum of the larva 

 and adult. In the larva the cerebrum is formed of (i) an unpaired 

 basal portion with a thin roof, and (2) of a pair of anterior lobes, 

 with small olfactory bulbs at their free extremities. 



The basal portion in the larva clearly corresponds in the 

 adult with the basal portion, together with the two posterior 

 cerebral lobes, which are merely special outgrowths of the dorsal 

 edge of the primitive basal portion. The pair of anterior lobes 

 have exactly the same relations in the larva as in the adult, 

 except that in the former the ventricles are prolonged for their 



