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 margi'nals) 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 minutissi- 

 mum, in which there are 54 rows of teeth. The typical pyg- 

 m&umhas 114 rows of 19-1-19 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 PYGM^UM 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: Large, scarcely able to withdraw into its she}l; 

 foot very broad; eye-peduncles but little developed; tentacles 



