ON CLAY 215 



as scattered individuals here and there, and bring with them new 

 conditions and animal forms. 



a) The subterranean- ground stratum. In addition to Pyramidula 

 mentioned above, other snails appear, especially in the more moist spots 

 on the bank. These are Zonitoides, Polygyra monodon (Fig. 169), 

 and P. thyroides (Fig. 170). Centipedes (Geophilus} and millipedes 

 (Polydesmidae) become more numerous, while the spiders (Pardosa 

 lapidicina) (Fig. 168), the tiger-beetle larvae, and other soil-inhabiting 

 forms decrease. 



b) Field stratum. The field stratum of the shrub stage does not 

 differ strikingly from the preceding, as it consists mainly of plants of 

 the earlier stage scattered among the shrubs. 



c) Shrub stratum. Here we have the characteristic inhabitants of 

 shrubs. On the young aspens and willow are the larvae of the viceroy 

 butterfly (163). The common gall on the willow is the pine-cone gall, 

 caused by Cecidomyiidae (137). Beneath the leaves of the cone we have 

 found long slender eggs of some orthopterous insect (probably Xiphidium 

 ensiferum) (40, p. 428). We have no record of the nests of birds, but 

 many of the forest margin birds nest here (see pp. 274-75 and Table 

 LXIV, p. 277). 



4. YOUNG FOREST STAGE 

 (Fig. I 7 l) 



Shrubs and seedlings of trees become more and more numerous. 

 The sweet clover and most of the animals associated with it disappear. 

 Young trees, such as oak, hickory, hop, hornbeam, etc., grow and usually 

 give rise to a sapling forest. 



a) Subterranean- ground stratum. This stratum has all the characters 

 of the more dense and mesophytic forest ground stratum and largely be- 

 cause of the springy character of the bluff which supplies much moisture. 

 The woodchuck (Marmota monax) (142) sometimes digs in these banks. 

 In the open places in which small areas of soil are covered with only a 

 few leaves we find the larvae of the green forest tiger-beetle (Cicindela 

 sexguttata) (55, 151) which lays eggs in shaded places (Figs. 172, 173). 

 Under the leaves the snails, which were recorded in the younger stages, 

 and sowbugs are present. We find snails and slugs (Polygyra profunda 

 [Fig. 220, p. 237] and albolabris [Fig. 240, p. 243], Philomycus caro- 

 linensis [Fig. 231, p. 241]), which are commonly abundant in dense 

 woods. The Myriopoda are also more numerous and belong to different 

 species. Fontaria corrugate (Fig. 218, p. 237), which has the margins 



