782 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [36] 



different from those of ttie bony fishes, and, therefore, bears no genetic 

 relation to the latter. 



Lepidosteus alone seems to form an exception to this unvarying rule 

 among the other Ganoids. The olfactory nerve in Lepidosieus at first 

 passes into a tunnel- shaped osseous tube, formed by the alispheuoid. 

 At the posterior part of the orbit it quits this tube and passes close 

 beside the semicartilaginous, semimembranous interorbital septum; 

 consequently at this point its course is free in the orbit. At the ante 

 rior part of the orbit both nerves enter a very long cartilaginous double 

 tube, which corresponds to that portion of the long rostrum of this 

 fish belonging to the primoidal cranium. At first glance we seem to 

 have presented us here a method of development corresponding in 

 every sense with that seen in the majority of bony fishes, yet this is 

 by no means the case. As already stated, the fenestration of the lat- 

 eral wall of the skull in the nasal region of bony fishes begins at the 

 anterior part of the orbit, at the place where the bulbus olfactorius 

 occupies a near position to the olfactory mucous membrane, and which 

 leads to a marked separation of the same from the membrana olfactoria, 

 and to the lengthening of the olfactory nerve. In Lepidosteus this long 

 double tube, in which the nerves are contained, is to be considered as 

 the original direct continuation of the skull cavity ; therefore the devel- 

 opment of an interorbital septum in this fish cannot have come about 

 in the same way that it did in the bony fishes, nor can the necessity for 

 the origin of the olfactory nerve be looked for in this fenestration. This 

 nerve must have originally in Lepidosteus, as well as m the other Ganoids, 

 been contained for its entire length in a continuation of the brain case, 

 which was separated by a median dividing partition into two canals ; 

 subsequently the lateral partition in the posterior interorbital part of 

 this septum disappeared, and in this way the olfactory nerve came to 

 lie in the orbit. 



In the course of this essay it would have been quite an easy matter 

 for me, in more instances than one, to have pointed out the facts going 

 to show that quite a number of the various structures in the bony 

 fishes can be traced with tolerable certainty to Amia, and from this the 

 opinion naturally arises that the same will apply to all the organs, 

 and that Amia is in reality a direct ancestor of the family of Teleosteans. 



Por this reason I have the more eagerly seized upon the opportunity 

 to point out the conditions referred to above with respect to the devel- 

 opment of the olfactory nerve, in which particular Amia has decidedly 

 reached a higher degree of organization than certain osseous fishes 

 lower down in the scale. 



In this place I will not omit the consideration of the morphological 

 conditions of the peripheral olfactory organs of the Ganoids and Tele- 

 ostei somewhat more critically, and compare them with corresponding 

 conditions in the Selachians. 



