234 Annals of the Carnegie Museum. 



marine sunfish, Mola, reaching many pounds in weight. The "pacu" 

 is the most famous, and is one of the principal food-fishes of the 

 Indians of Guiana. Myleiis pacu reaches a length of two feet and a 

 weight of ten pounds. 



Many of the characins have a black spot at the end of the caudal 

 peduncle extending to and partly covering the base of the caudal. 

 The Serrasalminse and Mylinae have this spot, which develops nor- 

 mally in the young, but becomes modified with age. The portion 

 on the caudal peduncle fades and the caudal portion sends out arms 

 along the outer part of the caudal lobes, so that the spot becomes 

 < -shaped. The rest of the caudal may remain clear of markings, 

 although there may be a distinct increase in pigmentation with age. 

 In other species chromatophores accumulate either along the margin 

 of the fin, or they may form a band at some distance within 

 the margin. A shoulder-spot is also found in very many species 

 of characins. Usually it lies above the lateral line and at some 

 distance from its origin. It may consist of a general concentration 

 of chromatophores from neighboring regions, which in consequence 

 are left free from chromatophores, or the spot may take on a definite 

 form characteristic of the species. The development of this definite 

 form is probably a secondary manifestation. The cause of the 

 primary congregation of chromatophores on the shoulder may either 

 be due to positive chemotaxis causing the migration of the chromato- 

 phores to a definite point, or to negative chemotaxis causing them to 

 migrate away from certain regions. I have been inclined to attri- 

 bute it to positive chemotaxis, which causes the chromatophores to 

 aggregate over or near a pseudotympanum over the anterior air- 

 bladder. A triangular area of the sides over the first air-bladder is 

 free from muscles in the young, the skin and peritoneum forming the 

 only covering of the body-cavity. The spot forms near this delicate 

 membrane. But the fact, that there is frequently an accumulation 

 of pigment-cells behind the unpigmented area surrounding the spot, 

 is strong evidence that the migration of the cells is at least in part 

 due to negative taxis. 



In the Serrasalminae and Mylinae the shoulder-spot, if developed, 

 is diffuse, and lies just back of the opercular margin at the origin ol 

 the lateral line. It varies very greatly in different species, and even 

 in the same species it may be totally absent or well-developed. 



Small circular spots on the upper half of the sides, or over the entire 



