660 MR. R. H. BURNE ON THE ANATOMY OF THE [May 25, 



and corresponds roughly to that of the rosette. In some genera 

 the cavity is extended to an appreciable extent beyond the rosette 

 (especially posteriorly) forming an empty space (Nasal sacs, 

 Column I.), which in many genera undergoes further development 

 to form a definite sac or sacs accessory to the true olfactory 

 chamber. 



The accessory sacs can be separated for convenience into three 

 series. (1) A single sac directed anteriorly from either above or 

 below the rosette (Nasal sacs, Columns III. & TV.). (2) A single 

 sac directed posteriorly towards the orbit (Nasal sacs, Column II.). 

 (3) Two sacs (ethmoidal and lachrymal nasal sacs) with very 

 definite relations to the ethmoidal and lachrymal regions of the 

 face, and constant also in their point of entry into the olfactory 

 chamber above and below the hinder end of the rosette (Nasal 

 sacs, Column Y.). With regard to the two sacs of the third 

 group there is not much doubt that they are homologous in the 

 different genera in which they occur, but in the case of the sacs 

 grouped in Series I. it is very difficult to determine how far they 

 are homologous among themselves or to either of the sacs in 

 Series III. In some cases ( Orestias, Anableps, Ophiocejyhahts, Tra- 

 chinus) the sac is possibly homologous with the ethmoidal sac, 

 in others [Gastrosteus (Solger), Solea) with the lachrymal, having 

 regard to its position, and especially to its mode and point of 

 connection with the olfactory chamber ; but in other cases {Mer- 

 luccms, Zeus, Sjjliyrcena, Clarias) the protrusion is of too general 

 a character to render the homology anything more than very 

 doubtful. 



The backwardly directed sacs of the Salmons, Herrings, and the 

 Mackerel (Series II.) being little more than a general protrusion of 

 the hinder part of the olfactory chamber, should probably not in any 

 way be regarded as homologous with either of the sacs of Series III. 

 Their distribution is interesting, being restricted to fishes in 

 which there is a great development of mucoid tissue about the 

 eye and face, giving rise to a third eyelid against the base of 

 which the sac abuts. 



The two sacs in Series III. are found in their typical form, with 

 one exception, in the Acanthopterygii. The exception is Mugily 

 which it should be noticed is in Gunther's sj'stem of classification 

 included among the Acanthopterygii. Upon the other hand, the 

 presence of these sacs in the Pleuronectidse appeai-s to lend further 

 justification for the removal of these Fishes from close proximity 

 to the Gadids (Giinther) to the Acanthopterygii (Boulenger). 



The least specialized form of the olfactory organ is undoubtedly 

 the rosette-filled hollow found in most of the non-Acanthopterygian 

 fishes (Physostomi, Giinther). From this as a starting point we 

 may justly assume that the development of accessory sacs com- 

 menced with a general expansion of the parts of the olfactory 

 chamber around the rosette, such as we find in Motella, and in a 

 slightly more accentuated form in Gymnarchus, the Siluroids, and 

 Fistularia. 



