VISCEEAL ANATOMY OF THE CHAEACmiD.E. 59 



be restricted to that family *, though by Boulenger apparently recognized also in certain 

 Cyprinoids. Whether this character has any physiological significance as a means of 

 increasing the sensibility of the bladder to external influences, or has simply arisen as a 

 consequence of certain lines of growth, appears to be doubtful. But, in any case, it is 

 worthy of notice as a furtlier suggestion of affinity between the three herbivorous genera 

 Citharltius, Xeiiocharax, and Ditstichodus. 



The posterior chamber of the bladder usually exceeds the anterior chamber in leugth, 

 in a ratio varying from about 5 to \. A notable exception occurs in the case of 

 Serrasalmo, as will be sliown later. The following figures represent approximately the 

 ratios in the forms in which I have recorded measurements : — 



Macrudon 3:3 Prochilodus 3:1 



Curimattts 3 : "i Auostomus 3:1 



A testes nurse 3:2 Ictdfiijoborus 4. : 1 



S annostonius 3:2 Xeohorus 4:1 



Sarcodaces 2:1 Euynatliiclithijs .... 4:1 



Lebiasina 2:1 Pliago 4:1 



Leporinus 2:1 Xenocliaru.c 4:1 



Bryconaethiops .... 2:1 Atestes Kotscfiyi .... 4:1 



Atestes macrotepidotus . . 2:1 Disticfiodus 5:1 



Hydrocyon 3:1 Citharinus 5:1 



These ratios represent length, not capacity. They do not appear to carry any great 

 significance. Herbivorous forms, for example, are found at both ends of the scale. Even 

 two species of the same genus, Alestes, are seen to be similarly separated ; but Alestes 

 kotscliyi is altogether peculiar as regards its air-bladder, and should not perhaps have 

 been included in the list given above. Perhaps the most that can be said is that in the 

 Erythrinoids the ratio is small ; that in the herbivorous forms Xenocharax, Distichoclus, 

 and Citharinus it is large ; and that in the allied genera Ichthyohorus, Neoborus, Fhago, 

 and Evgnathiclithys the ratio is a high one. Also it may he noted that in this, as in 

 many other points, Sarcodaces stands near the Erythrinoids. 



In Alestes kotschyi a peculiar condition exists : the air-bladder extends backwards far 

 beyond tlie boundary of the body-cavity — as far, indeed, as the root of the caudal fin. In 

 this region it is slender and tapering, and lies between the caudal muscles to the right of 

 the interhaemals of the anal fin. Such was the position in the two specimens I examined, 

 but it is said to lie sometimes on the left side f . Other members of the genus are stated 

 to jiresent the same formation in different degrees. 



In the genus Serrasalmo, in which the body-cavity is short and deep, the air-bladder 

 presents some curious features. It is much shorter than the body-cavity, and is unique 

 among Characinids, so far as my observations extend, in that the anterior chamber is not 

 only longer, but altogether larger than the posterior chamber. In Serrasalmo piraya, 

 indeed, the latter chamber is reduced so far as to appear like a mere conical cap on the 

 end of the former, which is several times its caj^acity. In Serrasalmo humeralis the 



* "The Air-bladder and Weberian Ossicles in the Siluroid Fishes," Phil. Trans. B. vol. 184, 1893, pp. 295-6. 

 t lioulenger, ' Les Poissons du Bassin du Congo,' pp. 148-9 ; also Cuvier & Valenciennes. 



