[25] THE OSTEOLOGY OF AMIA CALVA. 771 



of the bulla acustica, so extensively and in some cases excessively^^ de- 

 veloped in the Teleostei. I wish to state once more particularly that 

 the canals intended for the arches, and bounded everywhere by cartilage, 

 join with the labyrinth niche. 



The anterior semicircular canal begins at the anterior upper portion 

 of the utriculus inlet, courses laterally forwards and upwards, makes 

 a turn in the vicinity of the postfrontal, running close beneath the 

 cartilaginous skull cover, to be partly seen through it posteriorly and 

 towards the median line, and finally terminates in the cavum cranii in an 

 opening above the vestibule of the labyrinth (labyrinth niche). The 

 outer semicircular canal takes its origin from the posterior portion of 

 the utriculus, courses laterally and backwards, is barely seen just be- 

 neath the hyomandibular facet through the cartilaginous side wall of 

 the skull, then proceeds backwards towards the median plane to find 

 its exit, in common with the origin of the posterior canal, on the hinder 

 boundary of the sacculus. During its course the outer canal approaches 

 tolerably close to the intercalare. The posterior semicircular canal be- 

 - gins, as already stated, at the posterior margin of the sacculus, courses 

 laterally backwards and upwards, comes almost in immediate contact 

 with the exoccipital, then turns towards the median plane, forward, 

 and makes its exit just above the vestibule of the labyrinth. 



The description of the membranous labyrinth can be briefly presented. 

 So far as I could convince myself from the specimens that were at my 

 command, and really Avhich were hardly suitable for a critical examina- 

 tion, it perfectly corresponds in its general structure with the labyrinth 

 of the Teleostei, as we have learned from the admirable investigations 

 made by Hasse.^'^ It is described still more in detail by Eetzius.^^ 



The relation of the labyrinth to the cavum cranii in Amia calva shows 

 a marked difference when compared with that of the Selachii. While 

 in the Selachians the cavity of the labyrinth seems entirely isolated 

 from the brain case, there exists in Amia and all other Ganoids and 

 Teleosteans a more or less broad communication between these cavities. 

 It would hardly be amiss if one would trace the causes of the varying 

 size of the intercommunicating fenestra between the two cavities to 

 the entirely disproportionate development and unfolding of the body of 

 the labyrinth in the higher fishes, which has finally led to a stinted 

 growth of the medial dividing wall of the same. The acusticus foramen 

 has been in all probability the starting point for the fenestration of this 

 wall. At least I think we are justified in assuming this from the posi- 

 tion of this foramen of the labyrinth in Amia (when it is nothing more 

 than the occurrence of absorption of the periphery of the foramen acus- 



36 In the Scopehis and Gmioatovia I find a very extraordinary development of the 

 bnllse acusticse. 



"C. Hasse, Anatoniisclie Studien, Th. X. Das Gehdrorgan der Fische. Leipzig, 1873. 



=**G. Eetzins, Das Gehoroi-gan der Wirhelthiere [Vertebrates]. Th. 1. Fische und 

 AmpMbien, page 35. Stockholm, 1881. 



