Baker — A Revision of the Limnaeas of Northern Illinois. 19 



tion, stones, etc. Also found attached to floating garbage, 

 such as decaying apples, vegetables, etc. 



Remarks: This is one of 

 our most common species, 

 and, excepting L. stagnalis, 

 is the finest and largest 

 Limnaea we have. It is 

 always characterized by a 

 long and attenuated spire 

 which is twice as long as the 

 aperture. In palustris the 

 spire and aperture are nearly 

 equal, and the shell is wider 

 in proportion to its length 

 than in rejiexa, and the lat- 

 ter is very rarely malleated. 

 There is great variation in 

 the attenuation of the spire, 

 some forms approaching var. 

 attenuata in having a long, 

 narrow, pointed spire ( PI. 1 . 

 f. 3). The figures well illus- 

 trate this variation. 



The animal is generally rather sluggish in movement, but 

 sometimes moves with considerable rapidity, especially when 

 feeding. The species is as widely distributed in the present 

 area as palustris. 



Dr. Howard N. Lyon has raised this species from the egg 

 and has presented the set showing age development to the 

 Academy. Considerable variation is shown in the form of 

 the shell, the young (12-16 weeks) looking very like L. pa- 

 lustris, the characteristic "twist" of rejiexa not appearing 

 until the 21st week. The measurements of the successive 

 stages are as follows : — 



Fig. 11. Radula of Limnaea rejiexa 

 Say. C, central tooth; 1, first lat- 

 eral; 11, 14, intermediate teeth; 18, 

 24, 29, 37, 39, eighteenth to thirty- 



ninth marginals. 



6 weeks. 

 12 

 12 

 16 

 16 

 21 



Length 2.00; width 1.50 mill. 

 « 5.00; " 2.75 " 

 " 10.00; " 5.00 " 

 " 1300; " 6.00 " 

 " 20.50; " 7.50 " 

 " 21.50; " 9.00 " 



II 



This set shows that 

 some individuals grow 

 faster than others. 



