752 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [6J 



dition of these bones tliat supports this statement, viz, their relation to 

 the mucus canals of the head.^^ 



Among the Teleostei the anterior branch of the mucus canal, imbed- 

 ded in the frontal bone, begins with an opening which is situated to 

 the inside of the anterior nasal aperture. Its course iu the nasal is 

 backwards, and then it passes through the frontal, in which it throws off 

 several side branches. 



This portion of the mucus canal bears exactly the same relations to 

 the bones in question in Amia as in the nasal among the Teleosteans, 

 as may be seen by referring to Plate I, Fig. 1. 



The mucus canals can also be utilized in defining both lateral bones. 

 The main branch of the mucus canal, imbedded in the same, unites with 

 the canal of the suborbital arch, and only a small lateral branch anas- 

 tomoses with the mucus canal of the frontal. This condition reveals 

 the fact that the bone just mentioned must be the first piece of the sub- 

 orbital arch somewhat removed from its usual position — the antorbital. 



The middle uon-parial piece can also be determined without difficulty. 

 In it we see a rudimentary ethmoid which has abandoned its customary 

 site and relations with the frontalia, owing to the unusually developed 

 nasal bones. So Bridge has likewise considered it 5 in fact, one could 

 hardly regard it in any other light, unless choosing the very improba- 

 ble assumption that the ethmoid — very constant elsewhere — is entirely 

 absent in Amia, and that this fish is provided with a peculiar prenasal 

 bone that never ocQurs in other fishes. Our determination is undoubt- 

 edly correct, as we find in Polypterits an identically similar bone, 

 though here it is connected with two small processes of the frontalia 

 that enter in between the nasals.^^ 



All of the bones just described that overlie the cranium, with the sin- 

 gle exception of the prefrontal, are pierced by a system of mucus canals, 

 which are worthy of a closer consideration (see Plate I, Fig. 1). 



As already mentioned above, a large mucus canal commences, mesiad, 

 by the anterior nasal aijerture to follow a course first in the nasal, then 

 through the entire length of the corresponding frontal, to terminate in 

 the extreme anterior portion of the parietal, on the surface of which its 

 mouth is to be found. 



The right and left canal are connected anteriorly by means of a trans- 

 verse anastomosis which passes through the ethmoid. During its course 

 through the posterior part of the frontal the mucus canal just described 

 throws off a lateral branch, which passes through the postfrontal, and, 

 being confined between the bones of the orbital arch, passes around the 



" I desire to mention, at this point, that hitherto the relation of the mucus canals 

 to the hones of the cranium have hardly heen given a thought, and yet they deserve 

 a closer study, as these relations are very constant, and in questionable cases they can 

 be used to determine doubtful homologies. 



12 Cf. the representation of Miiller, Structure and Limits {Gremen) of the Ganoids, 

 PI. I, Fig. 1. 



