276 THE CHICAGO ACADEMY OF SCIENCES. 



of the peristome and body whorl. Length of foot 5.00, width 3.00 

 mill. Portions of the body frequently appear pinkish through the 

 shell. When the animal is drawing in air through its siphon, it will 

 withstand quite a violent jar before closing the respiratory orifice. 

 In a number of experiments, no instance was noted in which water 

 entered the respiratory cavity owing to these violent shocks. 



JAW : About three times as wide as high, much arched, with 

 rounded ends and a wide, flatly convex median swelling. (PI. VI, 

 fig. F.) 



RADULA (plate VII, fig. E) : Formula:J\+3^+| + i + f + 3_ 2 _ 3 +JJ T 

 (26-1-26) ; central tooth with a rather pointed cusp; lateral teeth tri- 

 cuspid, with a subquadrate base of attachment, the reflection very 

 broad, the entocone short and rather small, the mesocone long and 

 wide, reaching below the lower margin of the base of attachment, the 

 ectocone large and placed rather high up on the reflection; the ninth 

 and tenth teeth are modified laterals and. are intermediate between 

 laterals and marginals ; they are rather narrow with two unequal 

 cusps at the distal end and a third cusp placed higher up on the re- 

 flection ; a small cusp is frequently developed very high up on the 

 outer side of the reflection; marginals at first long and narrow, four 

 to seven cuspid with one or more small denticles high up on the outer 

 side of the reflection. The cusps vary widely in number, size and 

 position on the marginal teeth, but are usually four in number ; extreme 

 cuter marginals narrow with four or more denticulations at the distal 

 end. The examination of a small, narrow form of obrussa from Des 

 Moines, Iowa, gave 25-1-25 teeth, the intermediate teeth beginning 

 with the seventh tooth, the ninth tooth being a true marginal. A 

 specimen from Maine had seven laterals, the marginals beginning 

 abruptly with the eighth tooth. Obrussa seems more variable in the 

 number and position of the lingual . tooth than any of the other small 

 Lymnaeas. There are more than seventy rows of teeth. 1 



GENITALIA (Plate XI, fig. C) : Male organs: Penis-sac cylin- 

 drical, long and narrow, 2.00 mill, long, 0.75 mill, wide; penis very 

 long and slender, 2.00 to 3.00 mill, long, or as long as, or somewhat 

 longer than, the penis-sac; vas deferens 5.00 mill, long; vas deferens 

 to prostate 1.00 mill, long, entering the prostate without marked en- 

 largement ; prostate small, rather short, flattened, somewhat long-ovate 

 when viewed from the front, and very elongate-cylindrical when seen 



lr The radula described and figured by the writer in Mollusca of the Chicago 

 Area, page 267, is erroneous. The specimen from which the radula was ex- 

 tracted was not preserved, and it is now impossible to determine just what 

 species was represented. 



