508 



MR. N. A. MACKlNI'OSn ON Till!: CUONDIIOCRANIUM 



base of tliis triangle consists of a thickened plate representing the 

 originally thin ethmoid plate. Above and continuous with this 

 lies the nasal septum which expands posteriorly to foriri a vertical 

 pillar, the antorbital planum. The upper part of the antorbital 

 planum tvirns backwarils in the form of two lateral horns, the 

 ta3nia3 marginales (text-fig. 8) approaching the extroniities of tiie 

 epiphysial bar and postorbital processes presently to be described. 

 Tiie antorbital planum is pierced on either side by the foramen 

 for the olfactory nerve. The ethmoid structure has evidently 

 arisen from a median ridge and posterior upgrowth on the 

 ethmoid plate, the former giving rise to the nasal septum and the 

 latter to the antorbital planum. It is probable that at a slightly 



R.A.P 

 Stage 4. Lateral view of cliondrocraiiium. 



earlier stage the t:enicB marginales were connected with the 

 epiphysial bar, and afterwards ])ocame sopjuiited as is the case in 

 Gasterosleus {iiwhmevtnn, lUO'i). 'IMiis point will be referreil to 

 again later. 



The fused trabeculse are now hardly recognisable as such. 

 They are represented by two median pieces of cartilage, one of 

 which is attached to the ethmoid region wliile the other is a small 

 piece of free cartilage level with the posterior border of the eye. 

 The former remnant of the trabeculfB consists of a thickened base 

 resulting from the fusion of the two originally separate elements, 

 bearing a plate-like upgrowth which constitutes an interorbital 



