202 



LECTUEE XL 



ON THE STRUCTURE OF THE SKULL. 



THE SKULLS OF FISHES AND AMPHIBIA. 



C. The cranium, consisting chiefly of cartilage and without 

 cartilage bones, but ivith superadded membrane bones. 



The skulls of the chondrosteous Ganoids, the Sturgeons, and 

 Spatularise exemplify this type of structure, which forms a most 

 interesting transitional link between the skull of Plagiostomes 

 and the skull of ordinary osseous fishes. 



Spatularia has a completely cartilaginous skull, produced in 

 front into a great beak, flattened from above downwards. The 

 cartilaginous representatives of, at fewest, seven of the anterior 



Fig. 81. 



An Ji 



Fig. 81. — Side view of the skull of Spatularia with the anterior (asc) and posterior (psc 

 vertical semicircular canals exposed. — An, the auditory chamber: Or, the orbit with 

 the eye ; N, the nasal sac ; Hy, the hyoidean apparatus ; Br, the representatives of 

 tin; branchiostegal rays; Op, the operculum ; Mn, the mandible. 



