[39] THE OSTEOLOGY OF AMIA CALVA. 785 



posterior uasal aperture iu osseous fishes with the posterior nares of 

 the air-breathing vertebrates. 



A comparisou extended to a greater number of forms and the history 

 of development clears up the actual state of affairs in this case also, 

 and demonstrates that it is but an interesting case of "converging de- 

 veloiDmcnt" ['■'■ Iwnvergenten entwicMung-']. The position held by those 

 Teleosteaus which permanently possess but one nasal aperture on either 

 side, as for example Belone, the Pomaceutrides, many Chromides, &c., is 

 only to be determined with absolute certainty when we have a knowl- 

 edge of the history of their development. If one, however, considers 

 that the nearest kin to these fishes (Cyprinodonta, Labroidte) exhibit the 

 ordinary conditions, it will hardly be out of place to simply assume 

 that the dividing dermal bridge between the nasal apertures in the form 

 referred to has been secondarily reduced. 



As in so many other structures, so iu those of the uasal apertures, 

 the lowly organized Selachii prove to be the starting point from which 

 two diverging series can be traced ; upon one side the higher fishes, on 

 the other the air-breathing vertebrates. 



As I have already mentioned, the anterior and posterior nares in 

 Ainia are far apart, and, consequently, Amia represents a form that 

 must, as compared with the ordinary bony fishes, be accepted as pos- 

 sessing a higher state of development. The nasal hone is imbedded in 

 the broad dermal bridge between the two nostrils. Under these cir- 

 cumstances it is not at all strange that, in spite of tlie careful search I 

 made for it in this fish, I could not find the trace of a nasal alar carti- 

 lage in the vicinity of the nostrils. The nasal has taken upon itself the 

 original function of the same, that is, to support the entrance to the 

 nares, and thus rendered a nasal alar cartilage superfluous. 



To conclude the present article it only remains for me to draw a com- 

 parison between the cranium of Amia and that of the Selachii, with 

 which it may best be compared, and to liarticularize their resemblances 

 and their differences. Taken as a whole the latter are fewer in number 

 than one would at first suppose. The fundamental difference between 

 the skull of Amia and that of the Selachians rests upon the appearance 

 of the large connecting ossifications in the former. These ossifications 

 either simply overlie the primoidal cranium, or they are connected very 

 intimately with it, and without changing their form, replace structures 

 in it that were originally cartilaginous. 



The first appearance of the larger uuicing masses of osseous tissue 

 among fishes denotes one of the greatest and most far-reaching steps in 

 the progress of the process of development of vertebrate animals. It 

 indicates the first appearance of a tissue that, as a protective and sup- 

 portiiig material, proves far more suitable than cartilage. A glance at 

 a series of skulls of Selachians and Teleosteaus will be sufficient at once 

 to demonstrate the great significance of this "occurrence," 

 p. Mis. 67 5Q 



