458 
HENRY F. NACHTRIEB 
bill Figs. 6 and 7 show in detail the grouping in a small area 
respectively on an operculum and the dorsal surface of a bill. 
Careful examination discloses the fact that each dot represents 
at least one small pit surrounded by pigment cells, which to the 
unaided eye appear as one black mass. Toward the free edge of 
Figs. 6 and 7. Camera lucida sketches showing the grouping of the primitive 
pores respectively in a small area on the ventral surface of a bill and on the end 
of an opercular flap. Drawn by Helen A. Sanborn. 
the operculum the pigment cells often are less abundant about 
the pits than they are elsewhere. In some of the groups of pits 
the pigment cells are almost wholly absent, and in such places the 
pits can be located only by close inspection. All the pits here 
noted were called primitive pores by Collinge,^ and they will be 
so termed in this article. For convenience I shall also refer to 
them as pits. There are from fifty to seventy-five thousand of 
these pits on the bill, head and operculum of Polyodon. 
1 The Sensory Canal System of Fishes. Part I. — Ganoidei. By Walter Edward 
Collinge. Q. J. Mic. Science, vol. 36, New Series, 1894. 
