sg 
39 
shells deducted 
2. Average weight, main groups, pounds per acre, 
1-3-ft. zone 
Channel 4—7-ft. zone 
1915 1920 | 1915 1920 | 1915 1920 
Upper lake 317 0 87 0 | 50 72 
Viviparidae 
° and Middle lake 26 0 57 Opa.) 48 0 
Pleuroceridae : | 
Lower lake 391 110 446 24 174 0 
Upper lake 76 56 88 12 6 12 
Sphaeriidae Middle lake 43 0 54 a 66 6 
Lower lake 3 3 28 0 1 0 
Upper lake 0.8 0 0.1 0 8.5 0 
Amnicolidae, 
Valvatidae, Middle lake 0 0 1.7 0 0.6 0 
ete. . 
Lower lake 0 0 27.4 0 8.6 0 
8. Average weight of total snails 
and per cent. decrease, 1915 to 1920 
1915 | 1920 Per cent. decrease 
Channel | 394 — ~ 56 85.7 
Upper lake 4—T ft. 175 fan 93.1 
1— ft. 64 84 31.2* 
Channel 69 0 100.0 
Middle lake 4—T ft. 113 1 99.1 
1— ft. 86 6 93.0 
| i me 
, Channel 394 113 Y fa les} 
Lower lake “4—7 ft. 501 24 | 95.2 
| 
1—8 ft. 184 0 | 100.0 
* Increase, 
