ECOLOGICAL PAPERS. 



129 



those of the other months. Only some of the most typical species 



are hsted. TABLE II. 



Quantitative Surface Collections. 



2 > ^ ''=' > )C' o 



Group Name. Scientific Name. ^ r; "^ r^ ^ 1=^- • 



Snail L^mnaea reflexa (Say) 1 3 23 78 15 1 



Pianorbis exacutus (Sav) 3 10 I 1 1 



Physa gyrina (Say) . . ." 11 12 11 



Segmentina armigera (Say)... 1 10 4 



Pianorbis trivolvis (Say) 9 



Sphaeridae Musculium truncatum (Lins.) . 2 13 



Isopoda Asellus communis (Say) 3 30 25 2 



Mancasellus danielsii (Rich- 

 ardson) 3 3 1 1 2 1 



Amphipoda Eucrangonyx gracilis (Smith) 3 4 18 2 



Branchipus 6 



Ephemeridae larvae 2 20 



Agrionidse larvae 11 1 



Corethra larvae 2 1 1 



Hydrochinidas 4 1 1 1 



Dytiscidae Dytiscus sp 2 1 5 2 3 1 



The table shows that the number of individuals present at the 



surface increases and decreases in much the same manner as the 

 number of species. This is best shown by the snail Lymnaea 

 reflexa, whose relative abundance gives data for an almost perfect 

 seasonal curve. Pronounced cases of seasonal change due directly 

 to breeding habits are shown by the Ephcmcridce and Agrionidce 

 larvae. 



In midsummer this lilypad region was the most thickly popu- 

 lated of any part of the pond, but earlier in the spring animals had 

 been most abundant at the bottom along the pond margin. Table 

 III shows the results of dredgings made at the margin during this 

 season of marginal dominance. These collections were made with 

 a dip net one foot in diameter, with all variable features made as 

 comparable as possible. Each number is the result obtained from 

 three consecutive dredgings. 



TABLE III. 

 Quaxtitati\"e Bottom Collections. 



Group Xame. Scientific XAitE. 5. '< o 



Snails Pianorbis trivolvis (Say) 5 



Pianorbis deflectus (Say) 5 



Segmentina armigera (Say) 3 11 



Physa gyrina ( Say) 1 



Sphariidas Musculium truncatum (Lins.) 7 8 



Isopoda Asellus communis (Say) 54 48 57 



Mancasellus danielsii (Richardson"). 20 13 1 



Amphipoda Eucrangonyx gracilis (Smith) 49 62 4 



Dytiscidae Dytiscus sp 1 1 



Leech Erpobdella sp 1 1 



Hydrophyllidae larvae 1 



