EIGENMANN: the freshwater fishes of BRITISH GUIANA 111 



teriorly. In the latter two genera the clavicle is free from spines, but the preopercle 

 is provided with a spine at its lower angle. Granted that the presence of a spine 

 was advantageous at this point, it is evident that natural selection might pre- 

 serve two different lines of individuals which varied in this direction. 



Mutilation. 



Mutilated specimens which would not have survived, if there were the close 

 individual selection and elimination which we have imagined, are not rare. 



A Potamotrygon evidenth^ had a piece bitten out of the side by a " Perai." An 

 Astyanax abramoides had a inutilated snout of long standing, although it was the 

 largest specimen of the species obtained. 



A Loricaria brunnea had lost the posterior portion of the body and had re- 

 generated a caudal fin. A Trachycorystes obscurus had suffered a similar injury 

 and regenerated the caudal. Several specimens of Charax gibbosus and Eigen- 

 mannia virescens had suffered a variety of injuries. A detailed study of the mutila- 

 tions in the gymnotid eels will be published in a monograph on the Gymnotidce, 

 which has been prepared by Dr. Max Ellis, and which will shortly appear in the 

 Memoirs of the Carnegie Museum. 



The Characins. 



This family will be fully dealt with in another place, but a few of the new 

 points brought out by the present expedition may be mentioned. 



More than half of the strictly fresh-water fauna of Guiana is composed of 

 characins. In all there are one hundred and fifty-one species. In all localities, 

 except Lama Stop-Off, the Georgetown trenches, Waratuk, the Packeoo Falls, and 

 the Rupununi and Aruataima, they form more than half the number of species. 

 It was found that some of the small species (Plate XXXIX, figs. 3-7) burrow on 

 the sand-banks like some of the North American darters, and that some species 

 actually fly for a short distance after scooting along the surface of the water for a 

 much longer distarice. The characins are thus found in the bottom below the 

 river and in the air above it. It has long been known that different members are 

 adapted to, and live in, all the regions between the two. 



The organs of flight, which have been the subject of study by one of my students, 

 consist of the enlarged pectorals and the hypertrophy of the attached muscles. 

 To furnish points of origin for the muscles the coracoids are enormously expanded 

 and united below into a sternum. The entire anatomy of the anterior half of the 

 fish has been modified to become adjusted to this peculiar structure. (Plate LV.) 



