MB. E. DAY ON THE HEBBIDAL ARGENTINE. 81 



cannot be taken as a means for the discrimination of the two so- 

 called species, which must be considered as one. 



There is one subject respecting the air-bladders of the fishes of 

 this genus which is of great interest, belonging, as they do, to the 

 family Salmonida?, wherein this organ is of the Physostomous 

 variety, but destitute of any chain of ossicles connecting it with 

 the internal ear, as seen in the true freshwater Cyprinidse, Cha- 

 racinidse, and Siluridse*. Valenciennes mentions that the museum 

 at Paris had received a very good example of Argentina silus, a 

 little more than a foot in length, from the Bergen Museum. He 

 supposed that it had been captured at a great depth, for its sto- 

 mach was inverted. This inversion of the stomach is observed in 

 fishes suddenly brought up from great depths ; and is known to 

 be caused by the pressure of the water being rapidly lessened or 

 entirely removed, causing the gases in its interior to expand and 

 either burst the air-bladder or force the stomach into the mouth. 

 I do not think this phenomenon has been observed in Physosto- 

 mous fishes, to which the Salmonida? belong, as the pneumatic 

 tube, which is pervious throughout life, acts as a safety-valve, 

 and would permit this rapidly expanding gas to find an exit by 

 the alimentary canal. This brings us to the question of whether 

 the Argentines are or are not Physostomi, the same as the 

 remainder of the Salmonida?. Valenciennes states that they 

 belong to the Physoclisti, as, so far as he could ascertain in three 

 well-preserved examples, no pervious pneumatic tube could be 

 detected. 



If the Argentines undoubtedly belong to the class of fishes 

 having closed air-bladders, it is an exceedingly interesting fact — 

 one, however, I have as yet had no opportunity of investigating. 

 The genus Salmo contains fish, some of which are anadromous, 

 others freshwater ; but their affinities are unmistakably marine. 

 And here we observe another link in finding Atlierina, one of the 

 deep-sea SalmonidaB, possessing a closed air-bladder smaller than 

 perceived in other genera of the same family, perhaps due to the 

 depths at which it resides. For were it large and of the Physo- 

 stomous type, probably it would be unable to keep it distended 

 with gas, as such would be pressed out through its pneumatic 

 tube, unless the same mechanism were adopted as we see in the 



* Physostomous fishes are mostly freshwater forms, having a chain of 

 ossicles as described ; or if marine, they are mostly surface-swimmers or littoral 

 species, with a tubular prolongation of the air-bladder instead of a chain of 

 ossicles. Physoclistous fishes appeal' to be, as a rule, marine or of marine origin, 



