780 REPORT OF COMMISSIONER OP FISH AND FISHERIES. [34] 



roof was replaced by a membranous one. So long as such organizations 

 exist and no intermediate forms are known to us between the primitive 

 structures seen in the Selachians and the relatively and already widely 

 differentiated organization of Amia, this view of Gegenbaur's must remain 

 an hypothesis ; an hypothesis, to be sure, that has much to support it. By 

 accepting it, the survival of the transverse canal of the Selachii is ac- 

 counted for in higher vertebrates, if nothing else, and one is not compelled 

 to advance the dubious proposition that there exists in Amia, and in Tele- 

 osteans descended from Amia, a canal beneath the cavum cranii, unique 

 in the sense of being without antecedents, and whose importance and 

 homology would be quite enigmatical. The olfactory region presents 

 for examination two spacious canals in the interior of the skull, run- 

 ning side by side, parallel and in an antero-posterior direction, which 

 are separated from each other by a broad cartilaginous septum, and 

 which end in the foramina olfactoria at the base of the nasal fossae. 

 In the canals, which are to be considered as the direct continuation of 

 the cavum cranii, are to be found the very thick and firm olfactory 

 nerves. They are composed of a strong neurilemma which surrounds 

 a fasciculus of nerve fibers, some seven or eight in number, but loosely 

 connected together, and among which, to all appearances, no anasto- 

 moses take place. 



In fishes, as we are aware, two types can be distinguished, depend- 

 ing upon the relations existing between the nerve center of the olfac- 

 tory organs and their terminal filaments. In one case the bulbi olfac- 

 torii of the olfactory mucous membrane lie close by, and are connected 

 with the fore brain by a long tractus ; a single olfactory nerve does not 

 exist in this case, but rather, on the other hand, quite a number of short 

 nerve fibers pass from the bulbus to the olfactory mucous membrane. 

 In the other case the bulbi olfactorii are connected with the hemispheres 

 of the cerebrum ami arise as long and true olfactory nerves. At first 

 sight it would appear as though the difference was not an essential one, 

 and as though the bulbus olfactorius was no integral part of the brain, 

 but simply a collection of ganglionic cells occurring in the course of the 

 fibers of the olfactory, and could occupy divers positions. That it is, 

 however, is clear when we see the typical, very characteristic, difference 

 between the stout olfactory nerve, provided with a firm neurilemma, 

 and distributed to the periphery from the bulbus, and the thin tractus, 

 enveloped only in the delicate pia mater holding a central position with 

 respect to the bulbus. This same fact was particularly dwelt upon by 

 Stannius,*^ that these two specified conditions as regards the position 

 of the bulbi olfactorii are always independently present, that there is 

 either a bulbus adjacent to the brain or one annexed to the olfactory 

 membrane; cases in which a centrally located bulbus occurs in connec- 



'^ Stannius, Das peripherische nervensyatem d. Fische, 1849, page 2. [Stannius, The 

 Peripheral Nervous system of Fishes, 1849, p. 2.] 



