286 THE CHICAGO ACADEMY OF SCIENCES. 
Length. Breadth. Aperture length. Breadth. 
9.10 4.25 4.10 ito) saan, LeAbKee 
8.10 4.10 3.10 tata Dog Ae Planulata 
5.50 3.00 3.00 HOON Te 3} 
9.00 4.10 4.50 2.00 “ Maine 
8.75 3.90 4.00 2.00) = e 
8.00 3.50 Baio 2/00) we _ 
8.50 3.50 4.00 200ees a 
11.50 5.50 Beep 2.60 Iowa 
9.50 4.50 4.10 Palla) 2 A 
8.50 4.75 4.25 200K as " 
9.50 4.00 4.75 20) a 
Types: Plica, Smithsonian Institution, one specimen, No. 119485; 
blanulata, Smith. Inst., six specimens, No. 118676; the types of erigua 
are not in existence. 
Type Locatity: Plica, Tennessee; planulata, White Sulphur 
Springs, West Virginia; exigua, Tennessee. 
ANIMAL: Similar to obrussa externally. 
Jaw: Not differing in general form from that of obrussa. 
RaputaA (Plate VIII, fig. C, D): Formula: 7%414 61146114 48 
(25-1-25); central tooth like that of obrussa; lateral teeth tri- 
cuspid, the reflection quite broad, the entocone small, the mesocone 
very large and the ectocone small; the laterals become narrower at the 
third tooth, the entocone approaching the distal end; the seventh tooth 
is intermediate, the entocone becoming larger, so that it and the meso- 
cone are subequal in size; the true marginals begin at the eighth tooth 
and are claw-like. The eighth tooth varies somewhat in the form of 
the denticulations. (See pl. VIII, fig. C.) In one specimen from Des 
Moines, Iowa, the seventh tooth possessed three small, very acute 
denticles on the upper part of the outer margin; this probably indi- 
cated a splitting of the ectocone; the entocone was very large and very 
acute. (Pl: VI, fig. 0.) Where are aver Sixty rows oi fees 
teeth of exigua are very uniform. The radula differs from that of 
obrussa in having but six laterals and in the true marginals commencing 
on the seventh instead of the eleventh tooth. In form the teeth of 
both radulz are very similar. 
GENITALIA: The genitalia of exigua are in all respects similar 
to those of obrussa. A typical measurement is as follows (No. 23292) : 
c Penis- Vas. Prost. Rec. Penis  Penis-sac 
Penis. sac. def. duct. sem. retractor. retractor. Shell. 
1.50 IPAS 3.25 70 1.25 otha} 1.00 15) 
There are two posterior and two anterior protractor muscles of 
the penis-sac. 
