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.) 
RAvuLa (plate VI, fig. E): Formula:7%+,%74+841484,7, 47% 
(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, lowa, 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 
Lymnzeas. There are more than seventy rows of teeth.t 
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 
ionger 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 
disea, maze 267. 1s erroneous, | tin specimen tcamiiNicn ns TeanienaeEe ee 
tracted was not preserved, and it is now impossible to determine just what 
species was represented. 
