402 



and 1923 (U. L. : ex-channel, 1923, 20; 1923. 9; M. L. : ex-channel, 1922, 

 27; 1923, 20). 



Points of exception that must be noted are, however, several, as 

 will be seen from what follows : 



First it is noted that the two species of snails of" the family Val- 

 vatidae {V. bicar'mata and V. tricariiiata) which made tip the sole rep- 

 resentation of grovip II as above defined in the extra-channel zones of 

 the upper lake proper in 1923 both came from very close to the west 

 shore at Rome, and so can not be regarded as standing for any ver\- 

 appreciable part of the extensive upper lake wide-waters. A single oc- 

 currence of an unidentified caddis larva was also noted in the haul from 

 the middle of the "Goose Pond", the shallow backwater to the northeast 

 of the upper end of the upper lake which is rather better protected from 

 sewage invasion than the open portions of the main lake, .\gain. in the 

 upper lake channel in 1923 the only two group II species taken were spt- 

 cies of Bryozoa {Phimatclla princcps, two varieties) which attach them- 

 selves to dead shells, and do not live in the mud as do typical bottom 

 species ; and both of these have previously been found in the Illinois River 

 to exhibit a considerable degree of tolerance to pollution if there is some 

 current present. One of them was taken in Spring Bay Narrows only, 

 and the other at Chillicothe*, at both of which places there is much more 

 current than in the lake proper. 



Analysis of the group II records from the middle lake shows, first of 

 the intermediate zone (stations within first 1500 feet of the mid-channel 

 line), that all occurrences were restricted to the lower third of the lake, 

 and to the westf side of the channel ; and that four out of the total of 

 seven group II species were taken between Long Shore Beach and Al 

 Fresco Park within 50 feet of the west shore. Of the three group II 

 species taken in the outer zone of the middle lake one was taken close to 

 the west shore opposite Mossville ; one 600 feet from the west shore oppo- 

 site the same station; while the third record (of Pliivmtclla princcps. var. 

 friiticosa, a current-loving form), from 1900 feet east of the mid-channel 

 line opposite Al Fresco Park, is most jirobably based on an error — i. e.. 

 mixing of material either in the field, the field laboratory, or at Urbana. 

 The amount found in the bottle was only a trace and could easily have 

 been stuck in the sieve at a neighboring channel station and released at 

 the other without having been noticed. Last, the sole representative of 

 group II from the middle lake channvl (I'ah-ata bicai'i)iatii var. iionncilis) 

 was taken in the middle of the channel in 20 feet of water in the same 

 southwest corner of the lake that housed the great majority of tlie ex- 

 channel representatives of the same group. 



The principal increases in number of group II species in lower lake 

 cross-sections over 1922 were not in the extra-channel zones, as in the 



* Includeti as an upper lake station for bettor comparison will) 1922 records. 

 when ChiUicothe was last station north in series. 



t Stations on tliis side are for the most I'tart closer to the sandy (west) bluff. 

 and are lietter washed at times of flood than the formerly largely hnishy lake 

 areas to the eastward of the steamboat channel. 



