THE CHONDROCRANIUM IN THE ICHTHYOPSIDA. 97 



ments of the cornua of the earlier stages. In the same 

 plane with the ethmoid plate and just behind its posterior 

 end, a flat antorbital process {anp) projects from the side 

 of the trabecula and, curving forwards, terminates directly 

 behind the backward projecting process of the ethmoid 

 plate. 



A broad nasal septum rises from the middle of the 

 ethmoid plate separating the two olfactory organs and 

 forming the anterior wall of the cranial cavity. Bars of 

 cartilage, which I have called the tectal cartilages (tc), 

 connect the antero-dorsal points of the trabeculae with the 

 opposite points of the septum, roofing over the olfactory 

 foramina and forming the bases from which curved pro- 

 cesses, laminaj cribrosee (e), extend outward, downward 

 and forward, covering over the posterior ends of the 

 olfactory organ. 



The quadrate (q), besides its fusion with the otic cap- 

 sule as already mentioned, now presents a well developed 

 pterygoid process (pt), a C3dindrical rod of cartilage 

 joining the main part of the quadrate at the point where 

 the ascending process meets the body. Its general direc- 

 tion is forwards, parallel with the trabecula as seen from 

 above, and downwards with a slight outward curve near 

 the tip which lies just below the ventro-median surface of 

 the orbit. 



Fifth stage. — Young Ambly stoma sixty-nine mm. 

 long. Gills entirely atrophied (Figs. 12-13). At this 

 advanced stage in the development of Amblystoma, ossi- 

 fication has occurred to so great an extent that large 

 portions of the older cartilages have disappeared, giv- 

 ing the chondrocranium a l)rolven and ragged appearance, 

 especially in the otic region. The median capsular wall, 

 the anterior end of the basilar plate and portions of the 

 occipital arch, and the trabecuUe are entirely replaced 



ESSEX IN'ST. BULLETIN, VOL. XXVIII 7 



