CITHAKINUS. 153 



lobules in a sac behind the fourth gill ; it was regarded by its discoverer as a structure 

 arising from the gill of the rudimentary fifth branchial arch or inferior pharyngeal 

 bone, and intended for breathing atmospheric air, a contrivance which enables the fish 

 to remain longer in muddy, insufficiently aerated water. 



The brain-case extends far forwards, there being no interorbital septum. Skull 

 with a large fontanelle extending from the ethmoid to the supraoccipital, interrupted 

 by a narrow bridge formed by the frontals above the alisphenoids ; the supraoccipital 

 is produced into a high crest directed upwards and backwards ; supraorbital bone 

 large ; parasphenoid bifurcating behind in two long and slender processes ; acoustic 

 bullae greatly developed. Vertebrae 44 (25 + 19) in G. citharus, 42 (24 + 18) in 

 C. latus ; ribs very broad at the base. 



The air-bladder is large, the posterior chamber about five times as long as the 

 anterior. A meatus between the dorsal and lateral muscles under the origin of the 

 lateral line, as in Distichodus. Stomach with very numerous pyloric appendages. 

 Intestinal canal about five times as long as the whole fish, forming several coils. 



These fishes are mud-swallowers, feeding principally on algse and diatoms. 



Three species are known from the Congo Basin and two from the Nile. These may 

 be easily distinguished by the following characters : — 



C. citharus. — Base of adipose fin shorter than its distance from the rayed dorsal, 

 which is composed of 17 to 20 rays; anal with 25 to 31 rays; scales 77-90 

 2Qi25, 17-21 between lateral line and base of ventral fin. 



C. latus. — Base of adipose fin longer than its distance from the rayed dorsal, 

 which is composed of 20 to 22 rays ; anal with 23 to 26 rays ; scales 63-71 

 jg^o, 14-16 between lateral line and base of ventral fin. 



1. CITHARINUS CITHARUS. 

 (Plate XXV.) 



Serrasalmus citharus, Geoffroy, Descr. Egypte, Poiss. p. 40, pi. v. figs. 2 & 3 (1809). 



Citharinus geoffroyi, Cuvier, Regne Anim. 2nd ed. ii. p. 313 (1829) ; Miiller & Troschel, Hor. 

 Ichthyol. i. p. 9 (1845) ; Cuvier & Valenciennes, Hist. Poiss. xxii. p. 95 (1849) ; GKinther, 

 Cat. Fish. v. p. 302 (1864), and Petherick's Trav. ii. p. 240 (1869) ; Steindachner, Sitzb. Ak. 

 Wien, lxi. i. 1870, p. 538 ; Boulenger, Ann. Mus. Congo, Zool. i. p. 93 (1899). 



Distichodus marnoi, Stein dachner, Sitzb. Ak. Wien, lxxxiii. i. 1881, p. 200. 



Citharinus citharus, Boulenger, Proc. Zool. Soc. 1905, i. p. 151. 



Depth of body once and four-fifths to twice and one-third in the total length, 

 length of head three to four times. Head twice to twice and one-third as long as 



