42 ILUNOIS BIOLOGICAL MONOGRAPHS [42 



the ventral ends of the otic capsules, have been described as the centres of the 

 prootics. Schleip (1903) says: "Die ersten Spuren von Verknocherung treten 

 bei einem Lachs von 24 mm. Lange auf : das Prooticum besteht hier aus einer 

 ineren und einer ausseren, den Knorpel in der Umgebung des Facialisloches ein- 

 fassenden perichondrialen KnochenlameUe; ein Zusammenhang beider an der 

 Randern des Foramen ist aber nirgends vorhanden, vielmehr entbehrt die 

 Wand des Facialiskanals selbst den Knocheniiberzug noch vollstandig (Siehe 

 Fig. 5). Die genannten Lamellen sind wie iiberhaupt alle perichondralen 

 Verknochenmgen auf diesem Entwickelungestadium homogen, scharf vom 

 Knorpel wie vom Bindegewebe abgesetzt und haben kein anderes Reliefe, als 

 es durch die Anlagerung an die Knorpeloberflache bedingt ist; wenige schmal, 

 spindelige Zellen bilden das Periost. " 



In the sahnon, however, the eye-muscle canal is formed by the inward 

 growth of the rectus eye muscles between the parachordalia and the cavum 

 cranii, a membranous bridge roofing the canal so formed, and supporting the 

 brain. The homology between Amiurus and Salmo in this region is easily seen, 

 despite the specialization in the latter. In both, the parachordalia are con- 

 tinuous with the otic capsules posterior to the hypophysis and there is the nar- 

 row fenestra basicranii anterius between them, the floor of which is formed by 

 the parasphenoid ossification. There are the same inner and outer perichondria! 

 lamellae as described by Schleip (1903), but in Amiurus at this stage these 

 lamellae have united. The 'processes of Comparetti' do not occur in Salmo, 

 but are limited to the Ostariophysi. The relations of these processes to the 

 cartilage of the floor of the cavum and to the cavum itself are very similar to the 

 eye muscle relations and have arisen by an invasion of the cranial floor. The 

 thin osseous wall of the otic capsule, described above, is formed in the same 

 way as in Salmo (Schleip) t by the disappearance of the inner lamella and resorp- 

 tion of the underlying cartilage, so that the wall is formed by the outer lamella 

 alone. 



In Amiurus, the cartilaginous wall (Fig. 4) lateral to the utriculo-saccular 

 connexion is very thick and is continuous dorsally with the septum semicircu- 

 laris laterale. That part of the parachordal which contains the sacculus forms 

 an angle of about 1 20 degrees with the otic capsule proper. The glossophar\Ti- 

 geal nerve issues from the cranium dorso-lateral to the sacculus and between it 

 and the utriculus (Fig. 4). The anterior margin of its foramen is formed 

 entirely of cartilage, (Fig. 4). Posterior to the passage of this nerve the sacculi 

 are again enclosed in recesses as were their anterior ends. The ossifications 

 which form the roof extends transversely from between the two jugular fora- 

 mina (Fig. 26) and meet by suture medially below the brain. From the ventral 

 surface of each of these ossifications, an ossified bar extends obUquely toward 

 the midventral surface of the basal plate. This basal plate is triangular in 

 cross-section and forms a cartilaginous crest between the recessus sacculonmi. 

 It is suimoimted by a perichondrial ossification which has two diverging proc- 



