I. INTRODUCTION. 



Explorations in the field of morphology are continually bringing to light new 

 facts which modify our ideas regarding structural features already recognized, and 

 also make it necessary to take account of new ones. This progress is the result of 

 intensive work with, of course, better instruments and better technique. One result 

 of this work has been to concentrate the attention within relatively narrow limits, so 

 that one observer often misses that which will lie in the path of another working on 

 the same material. Of this there is no lack of illustrations, and another case in point 

 is now found in the. brain of the selachians. From the time of Busch ('48) through 

 Leuret and Gratiolet ('39-57) to the publication of Fritsch's classical memoir ('78), 

 the brains of various selachians had been carefully examined and figured. From 

 Fritsch's time to the present the brains of Squalus acanthias and other selachians have 

 been among the most frequently examined nervous systems both from the standpoint 

 of structure and of development. In the study of the selachian head the present 

 writer also had a modest part ('95), but a pair of cranial nerves was continually 

 overlooked. I found them first in Acanthias embryos about six years ago, and, 

 published (Locy, '99) an account of their embryonic development in that 

 selachian. At first, very naturally, I looked upon these nerves as of exceptional 

 occurrence, possibly transitory in existence, and probably confined to a very limited 

 number of species. I have, however, continued to find them in all the different adult 

 selachians that I have had opportunity to dissect. In this paper it is proposed to 

 describe these newly observed nerves in six genera of adult selachians and their de- 

 velopmental history in one. Their occurrence in the adult stages of six genera is 

 sufficient to show that they are not transitory structures, nor are they confined to a 

 very limited number of species. 



It is a great pleasure to the writer to prepare this paper as an expression of high 

 regard for Professor Mark, under whose stimulating guidance he received his training 

 in morphological method and took his first steps in independent work. 



The nerves in question are connected, peripherally, with the olfactory epithelium 

 and terminate, centrally, in the upper part of the corpus striatum. One's first im- 

 pulse would be to look upon them as possibly representing a pair of bundles of the 

 olfactory system run wild, rather than as new elements in that system, or as inde- 



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