46 ILLINOIS BIOLOGICAL MONOGRAPHS [46 



again not related to the cartilage, and sometimes entirely wanting. We have 

 no reason to believe that at some stage in the phylogenetic history of the 

 vertebrates there was another ossification forming the wall of the cranium in 

 place of the otic capsules of today. Could we prove that the otic capsule 

 superseded an osseous or cartilaginous brain-case wall, only then could we say 

 that the otic bones were not parts of the brain case. 



The sphenotic is the most anterior dorsal of the otic bones and according 

 to those who have studied its development in the teleosts, it appears as an 

 ossification around the dorsal anterior end of the cartilage of the anterior 

 semicircular canal recess, and of a part of the posterior end of the alisphenoid 

 cartilage. In the 30 mm. Salmo (Schleip, 1903), the first sign of this ossifica- 

 tion is an osseous lamella in the perichondrium of the postorbital process, 

 which forms a ledge of bone projecting laterally above the hyomandibular 

 articular surface and extending posteriorly along the roof of the anterior semi- 

 circular canal. The levator operculi muscle is attached to its outer surface. 

 It spreads dorsally on the surface of the cartilage as far posteriorly as the 

 pterotic ossification (which is developed on the roof of the lateral semicircular 

 canal) and ventrally as far as the hyomandibula. At a later stage another 

 perichondrial lamella is formed on the inner surface of the roof of the anterior 

 semicircular canal. The formation of the adult bone takes place by the resorp- 

 tion of the cartilage between these two layers and by endochondrial ossification. 

 Schleip makes no mention of the development of a lateral line canal ossification 

 in connexion with the outer lamella. A fine nerve issues through the cartilage 

 and bone in this region to innervate the sense organ of the lateral line canal 

 contained in the pterotic ossification. 



In the development of the sphenotic of Gasterosteus (Swinnerton; 1902), 

 the inner lamella and the cartilage disappear, so that the wall is formed by the 

 outer lamella alone. 



In the 32 mm. larv^a of Amiurus, ossification is advanced, but I didnotfind 

 any ossifications in this region of the 20 mm. larva. The ossification enclosing 

 the supraorbital lateral line canal is intimately connected with the perichon- 

 drial ossification on the outer surface of the cartilage forming the roof of 

 the recess for the anterior semicircular canal (Fig. 19). Just anterior to the 

 cavimi of the anterior semicircular canal, the ramus oticus of the facial nerve 

 (the fine nerve of Salmo, Schleip, 1903) issues through the cartilage (Fig. 3) 

 and divides into anterior and posterior branches; the posterior continues within 

 the lateral osseous canal to innervate the next sense organ of the canal system. 

 The perichondrial ossification and that of the lateral line canal cannot be distin- 

 guished from each other. Although there was no ossification in the 20 mm. stage 

 in this region, the beginning of the lateral line ossification was represented by a 

 heavy tract of osteoblasts surrounding the membranous canal. With subsequent 

 ossification, the bone thus formed fused immediately with the underlying 

 perichondrial ossification of the aHsphenoid cartilage and the otic capsule. The 



