A NEW CRANIAL NERVE IN SELACHIANS. 53 



Pinkus describes the nerve in Protopterus as connected with the " Zwischen- 

 hirn," but in all the selachians I have examined it is connected with the forebrain. 



Allis ('97) describes a somewhat similar bundle in both young and adult stages 

 of Amia, which he believes to correspond to the nerve described by Pinkus. I have 

 had opportunity to examine this bundle in the adult Amia. In that form it does 

 not have the striking independence that I have spoken of as characterizing it in the 

 selachians. It is more closely united with the fila olfactoria, and is not nearly so 

 distinct even where it runs over the ventral surface of the brain. 



It appears to me now, after observing this nerve more widely in the selachians, 

 that it corresponds to the nerve described by Pinkus in Protopterus and by Allis 

 in Amia, and I believe the conditions justify calling it "a new nerve." Even if it 

 be one of the olfactory bundles in an unusual position, its separateness in origin and 

 differences from all other olfactory radices would still justify the use of the designation 

 "new nerve." 



In my paper of 1899 I suggested in a tentative way calling it " accessory olfac- 

 tory" in the following words: "On account of its close relation with the fibres of 

 the main olfactory and to the nasal membrane, it is best for the present to refer it 

 to the olfactory system and, perhaps, to designate it 'accessory olfactory.' We 

 need to know its central and peripheral terminations and whether it is represented 

 in other animals before saying much about its homology." 



This new nerve must not be confused with the thalamic nerve discovered in 1891 

 by Platt and Froriep, and whose history was so well worked out by Hoffmann in 

 1897. The thalamic nerve is between the midbrain and the thalamencephalon. 

 The new nerve and the thalamic exist simultaneously in embryos of Squalus acan- 

 thias, but the latter is transitory. 



I have looked with especial care for traces of this nerve in a number of amphib- 

 ians and teleosts. Both embryonic and adult stages have been examined in Nec- 

 turus, Amblystoma, the frog, the toad, the trout, catfish, etc., etc., but in none of 

 these has the nerve been found. It is to be hoped that further observations on a 

 wider range of material may bring us more facts regarding the existence of this nerve 

 in other forms, its history and relationships. 



