286 THE CHICAGO ACADEMY OF SCIENCES. 



Length. 

 9.10 

 8.10 

 5.50 

 9.00 

 8.75 

 8.00 

 8.50 

 11.50 

 9.50 

 8.50 

 9.50 



TYPES : Plica, Smithsonian Institution, one specimen, No. 119485 ; 

 planulata, Smith. Inst., six specimens, No. 118676 ; the types of exigua 

 are not in existence. 



TYPE LOCALITY: 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. 

 RADULA (Plate VIII, fig. C, D) : Formula : 1 T_Vhi+}+i+i+i+* 1 -V 

 (25-1-25) ; central tooth like that of obrussa; lateral teeth tri- 

 tuspid, 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 pi. 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.) There are over sixty rows of teeth. The 

 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 radulse 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) : 



Penis- Vas. Prost. Rec. Penis Penis-sac 



Penis. sac. def. duct. sem. retractor, retractor. Shell. 



1.50 1.25 3.25 .70 1.25 .75 1.00 5.25 



There are two posterior and two anterior protractor muscles of 

 the penis-sac. 



