10 ILLINOIS BIOLOGICAL MONOGRAPHS [10 



trabecula communis. This septum, by growth backward and dorsally, limits 

 the anterior extent of the cavum cranii. The bulbus olfactorius, which in the 

 larva of one of these forms, has the primitive cyclostome relation, is carried 

 posteriorly by the interorbital septum, so that it becomes enclosed within the 

 cavum cranii. The bulbus retains its connexion with the olfactory organ by 

 a long slender olfactory nerve which passes unenclosed across the orbit 

 between the anterior end of the cavum wall and the ectethmoid process. Sage- 

 mehl recognizes this condition as the teleost type of olfactory development. 

 It is also interesting to note that in the Characinidae (Sagemehl 1885), Cithar- 

 inus is very close to the selachian type and Macrodon has the teleostean rela- 

 tion of parts. Other members of this family present greater or lesser degrees 

 of relationship to the two types. The olfactory region at this stage of develop- 

 ment in Amiurus, according to this view, has just reached the cyclostome stage. 



The orbital region. Each lateral wall in the orbital region of the 10 mm. 

 Amiurus is formed by a ventral trabecular and a dorsal alisphenoid cartilage, 

 between which, the optic, oculomotor, most of the trigeminal, the abducens 

 and the facialis nerves pass outward (Figs. 1, 2). In front of the opticus these 

 two cartilages meet and form a soUd wall for a short distance, its continuity 

 being broken by the orbital foramen, dorsal to which the alisphenoid is fused 

 to the posterior face of the ectethmoid process. Posteriorly the alisphenoid 

 cartilage is fused to the anterior face of the otic capsule. A connective tissue 

 membranous wall connects the ventral margin of the alisphenoid cartilage with 

 that of the trabecula and it is through this membrane that the above men- 

 tioned nerves pass (Fig. 14). 



The ramus oticus of the faciaHs nerve passes ventro-dorsaUy through the 

 aUsphenoid cartilage, just anterior to its union with the otic capsule. At 

 about the middle of the cartilage are two small foramina, the more ventral for 

 the ramus ophthalmicus superficialis of the trigeminus and the more dorsal 

 for the ophthalmicus superficiaUs of the faciaUs (Figs. 1, 2). After its exit 

 from the cranium and its passage through the orbit, the ophthalmic branch of 

 the trigeminus passes through the orbitonasal foramen of the ectethmoid proc- 

 ess, mentioned above, while the ophthalmic branch of the facialis passes 

 around the lateral margin of the process. 



A cartilage bar, the epiphysial bar, described by Sagemehl (1885) in the 

 adult Characinidae and Cyprinidae, and by Pollard (1895) in the Siluridae, 

 extends across the dorsal surface of the forebrain just posterior to the ecteth- 

 moid processes and is fused at each end to the anterior dorsal ends of the alis- 

 phenoid cartilages. There is no other trace of a cranial roof in this region at 

 this stage. 



The alisphenoid cartilage was described by Sewertzoflf (1897) in the larval 

 Acanthias as a large cartilage lying lateral to the fore- and mid-brains and con- 

 nected secondarily with the anterior parachordaUa. Later it fuses behind with 



