Dr Herdman on the " Olfactory Tuhercle.'' 255 



olfactory tubercle is placed, is often of considerable import- 

 ance in specification, and consequently it is convenient to 

 have a name for it. I intend in my descriptions to refer to 

 it as the " peri- tubercular area." 



The olfactory tubercle is always situated in the peri- 

 tubercular area, which is an offshoot from the zona prse- 

 branchialis. The tubercle in its simplest form, as described 

 by Kupffer,* consists of a pair of parallel ridges separated by 

 a groove, becoming continuous with each other at their ends, 

 and bent so as to describe a curve. In the more complex 

 forms the essential structure remains the same — two ridges 

 continuous at their ends, and separated by a more or less 

 narrow groove. The curves into which this band may be 

 twisted, and the resulting shapes of the organ, seem in- 

 finite — thus necessitating the determination of how far certain 

 differences are specific characters, and how far merely indivi- 

 dual variations. 



Even when first described, this organ was considered a 

 specific character, as Savigny, in his " Tableau Systematique," 

 refers to the condition of the olfactory tubercle in the de- 

 scriptions of several of his species. After Savigny's time it 

 became customary not to discuss the condition of the bran- 

 chial sac and other internal organs, but merely to describe 

 the external appearance of the animal, and it is only of late 

 years that systematists have recognised the necessity for an 

 account of the internal anatomy, as well as of the external 

 appearance, in the proper description of a species. The 

 olfactory tubercle has again come into use as a diagnostic 

 character, and its condition in different species is found in 

 the descriptions of Lacaze-Duthiers, Kupffer, and Traustedt. 



That certain shapes of the olfactory tubercle are charac- 

 teristic of certain species is undoubted, and I believe also 

 that it would be proper to add this organ to the list of 

 characters given for groups of a higher rank (genera and 

 families), as in some of these, certain general types of tubercle 

 seem to be characteristic. In addition to these points, I wish, 

 however, to lay special stress on the fact that, while varying 



* Knpffer — II. Jaliresbericlite d. Komm. z. Untersiich. d. deutsch. Meerc, 



Kid, 1874, p. 201. 



