No. 645] 



VERTEBRA OF FISHES 



361 



gards " race." It was all obtained in a lagoon in Jack- 

 son Park, Chicago, during the third week of December, 

 1919. At this time what seemed to be the entire fish 

 population of the lagoon was congregated in an opening, 

 about five meters wide, in the ice along shore. These 

 fishes showed symptoms of asphyxiation. They were so 

 abundant that at times, while they were gyrating about, 

 the mass of fishes below would force the almost solid 

 upper layer a centimeter or two above the surface over 

 an area of perhaps a square meter. A water bucket was 

 filled with fishes, mostly Notropis atherinoides, by two or 

 three sweeps of a small hand-net. More than one thou- 



sand of the young of that year (1919) of the Notropis 

 were saved after random selection, and preserved for 

 study with all older fish of the same species. All of the 

 sunfishes {Lepomis incisor) obtained at the same time 

 and place were preserved and studied. Of the two spe- 

 cies, the sunfishes belonged to a population lu-aetically 

 confined to the lagoon, while the minnows hnd mo\ (H\ into 

 the lagoon, late in the preceding autiunn. fioiii tlie more 

 open waters of Lake Michigan. 



The specimens tlius obtained wei-c <zinii[rc(l iiit<i Ncar- 

 classes. Age dotoriniiialioiis were iiintif hy the usual 

 methods of couiititm' the ;iiiiiuli iwint.'r line-) on the 

 scales, and as a clirck Ww M^aMui.-il l.;nui> nf liir nt.^litlis, 

 and furthermore by the i)rei)arati(.n of n \'vv<iuvucy urnph 



