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216 THE CHICAGO ACADEMY OF SCIENCES. 
tion large, bicuspid, the inner cusp wide and reaching more 
than half way to the lower margin of the base of attachment; 
outer cusp short, wide, widely separated from the inner cusp; 
the outer laterals and marginals are similar to the first lateral, 
excepting that the inner cusp first becomes longer, and then 
(on the extreme marginals) becomes equal in size with the 
outer cusp, and the base of attachment becomes short and 
wide. This description is from the form known as mznutissi- 
mum, in which there are 54 rows of teeth. The typical pyg- 
maum has 114 rows of 1g-I-I9 teeth (Fig. 53). All of the 
teeth are more or less separated. ; 
Distribution: Circumpolar. Northern United States and 
Southern Canada south to Texas and west to California. 
Geological distribution: Pleistocene. 
Habitat: Found rather plentifully under chips and pieces 
of wood on the edge of forests. 
Fig. 53, 
Radula of PUNCTUM PYGMUM Drap. (MINUTISSIMUM Lea.) c, } 
central tooth; 1, first lateral; 8, transition tooth; 12, marginal tooth. 
Remarks: This is one of-our smallest shells, and is easily 
distinguished from all others by its subglobose form and dis- 
coidal whorls. Thus far it has only been collected in the 
northern and western regions, but it will probably be found, 
after careful search, in the southern region. 
Heterurethra. 
Superfamily Elasmognatha. : 
Jaw provided with a superior, quadrangular plate. 
FAMILY SUCCINID. 
Shell: Thin, transparent; aperture very large; spire very 
small and short. 
Animal: WLarge, scarcely able to withdraw into its shell; 
foot very broad; eye-peduncles but little developed; tentacles 
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